TLDR: This is a great upgrade, but if you have an older car, you will likely need/want to replace some other OE parts while you do the install, especially if you have an older car. The final fitment will be more crowded than the OE inlet, and you'll need to do some maneuvering and improvisation to get everything to line up properly, but that is the intended design/fit. If you want OE fitment, I don't think it's possible without removing the intake manifold.
You have to hack up the OE inlet to get it out. So be prepared for that. There's really no going back once you've committed. I think you'd also be hard-pressed to get a replacement OE inlet back in without removing the intake manifold.
While removing the old inlet, I identified some things I wanted to replace before putting the new inlet it and getting everything put back together. My car is over 20 years old, and in addition to breaking some old and brittle vacuum lines, I also took the opportunity to replace the PCV valve (stayed intact, but the hoses were rock hard) and the throttle body gasket (which looked really brittle), since I had easy access to those once I had torn everything down to get the old inlet out.
My OE inlet was pretty beat up where it joins with the turbo. It was super mushy and melted, probably from 20+ years of small amounts of oil cycling through, and eventually settling in a pool and eating away at the inlet from all of the hot/cold cycles. I've included a photo of the OE inlet, which probably tore a little bit while I was removing it, but I'm also 99% sure it was already leaking, which is why I wanted to replace it in the first place.
I also broke a few really old and brittle vacuum lines, had to literally chip away at the old PCV valve hose assembly with a hammer and chisel to get the last little bits free from the block, my throttle body gasket basically broke a little bit every time I touched it, and the valve purge on the evap system broke in half when I removed that vacuum hose from the OE inlet. I've included a couple of photos of everything that broke/had to be replaced. All-in, I spent $200 on OE replacement parts at my local Subaru dealership, which isn't too bad IMO. If you have an older car, make sure you budget some extra time and money to 1) properly replace brittle parts that break during the removal/install and 2) replace some things like the PCV valve and throttle body gasket while you have access, even if they seem serviceable.
The crux of the install is getting the Perrin inlet around the turbo. It comes in at a bit of a funny angle, and is compressed a little bit, and the underside and passenger side are basically inaccessible to try to maneuver the lip of the inlet. I ended up taking a small cord, looping it inside of the inlet, and then pulling both ends in opposite directions around the outside of the inlet to draw the lip of the inlet out around the turbo from the inside. Once it's on, you need to twist and pull and squish and test fit all of the fittings and hoses before you tighten everything down. You will need to figure out orientations for everything that cause the least tension and pressure, but things will be touching.
I have this paired with a Cobb intake, airbox and post-MAF hose, and the final fit for everything upstream of the inlet is a little bit snug, but totally fine.
The quality of the Perrin inlet is great. The fitment is challenging and snug, but definitely acceptable. It solved my leaky inlet issue, and I'm happy to have replaced some other old critical parts that weren't in good shape.
