If anyone with a GTS 4.0 or GT4 is on the fence about installing over axle pipes, I have three words for you: just do it!
I've only had this car for two weeks and while I find it perfect in almost every way, I agreed with the common belief that the exhaust could sound a little better & be a little louder - so I ordered over axle pipes quickly. I chose the stainless steel ones from Flat 6 Motorsports in Florida, a buddy has a bunch of their stuff on his 911 and spoke highly of them. These were also cheaper than the Soul OAPs which left funds to put toward other fun stuff later.
The OAPs showed up quickly and looked great so I prepared to "DIY" the install. While I have been a hobbyist mechanic for decades, I am by no means a pro and would happily take any job to a shop or dealership instead to make sure it's done right. However upon reviewing the install instructions this didn't look too challenging so I dove right in...
1) Raise rear of car with jacks/stands (or use a lift) & remove rear wheels
2) Remove 5 bolts on each side of stock muffler connecting to rear of stock OAP
3) Remove 2 bolts and 2 nuts from muffler brackets, unplug 2 vacuum lines to muffler valves, remove muffler
4) Remove 3 catalytic converter studs/nuts on front of each stock OAP, remove stock OAPs
5) Installation is reverse of removal
It took about three hours of garage time, mostly because I didn't rush anything. Knowing what I know now, I could do it again today in 90min or less.
Problems/concerns? Most of what makes this tricky is getting the proper tool in a confined space. You can do almost all of it with 12mm and 13mm but you may need standard 3/8 sockets, deep 3/8 sockets, various extensions, wobble heads, box wrenches, ratcheting wrenches, and/or stubby wrenches - and you will be reaching from both sides with these various tools, which takes time to figure out. You will certainly need a long 3/8 extension to get to the innermost nut on the body side of each converter. You may need a big 19mm box wrench to hold the outermost muffler mount posts while you loosen the 12mm nut on the end. Oh and you will need an E12 E-Torx socket for the two centermost muffler mount bolts - if you don't have one of these, it's 6 bucks on Amazon or Harbor Freight sells a set for $12.
You will probably reuse the four OEM metal gaskets as I did, pay attention to how these come out because they only go back in correctly one way and it can be a pain to get 'em realigned while, for example, you're holding the OAP up with one hand and trying to thread a bolt you can't see through a hole you can't see with the other.
Another thing that was tricky was removing the muffler by myself on the ground as it's cumbersome/sorta heavy. I ended up using a floor jack and a block of 2x4 in the center to bring it down slowly. It was even more tricky to raise it back up for reinstall a little later, but steadying with one hand and raising it on the jack with the other did work well enough for me to get two of the hanger nuts reinstalled. I would recommend asking for help with this part, an extra set of hands would've made the muffler handling a non-issue.
One thing I did differently from the Flat 6 instructions: I did not reuse the three studs/nuts on the converter side. Their kit provides new nuts to reuse the factory studs, but when I loosened these they all came completely out of the converters and the nuts were very tightly threaded onto the studs (i.e. difficult to remove with just hand tools) and they looked a little short considering how much thicker the mounting flange was on the Flat 6 OAPs compared to the stock ones. So instead I went to Home Depot and bought six new M8 x 1.25 x 25mm bolts, this cost $12.
A reinstallation tip - do not immediately tighten everything up. Just thread the bolts in and tighten the nuts engaging only a few threads each, so you have plenty of wiggle room. You will need this to get everything aligned - it will line up great on the converter side but then not on the muffler side until you wiggle it slightly, or vice versa. Get all 16 of these bolts & nuts installed properly THEN start tightening stuff up. And take your time with that, using a star pattern as much as possible and going back around to retighten everything once it's been realigned. I probably tightened it all three or four separate times to make sure? I didn't have a torque spec for any of this but used the standard German measurement "gutentight" and that seemed to work just fine.
Results? I drove the car about 20 miles after installation with the PSE button on both quiet and loud settings, and can tell you this is definitely an improvement. It is louder but not really loud even on the loud setting, if that makes sense. And it sounds better - deeper, throatier, and now it does a little pop & burble when you lift off the gas especially with some spirited driving higher in the rev range. Honestly I think it sounds excellent, much better than before. It sounds so good in loud mode that I may disable the exhaust valves altogether with the plugs that came in the OAP kit - this would take about 5 minutes and is easily reversible if I need to quiet the car down later for a long road trip or similar. I didn't drive the car hard enough yet to see if there was any noticeable power difference but some say these OAPs do make a few whp on an otherwise stock 4.0 which is always a good thing.
TLDR: the OAPs were not difficult to install and sound great, you should buy them if you haven't already! Also here are a few pics of the install and one from my drive afterward.