Golf sport wagen and not GTI, but some of my experience:
Picked up mine awhile ago and got around to installing it this month. So far so good (3ish weeks and counting), things seem to be happy. I'll be running some logs when I get a chance, but things have been hectic so that's been put off for a bit. Some notes: Their instructions kinda suck with the order they have things listed. Reading through them a few times to familiarize yourself is recommended. Others have used youtube for vids posted of the arm install there, and claim they help quite a bit more.
Ambient air sensor has you grind/break or bend the OEM tab on the crash bar - not sure if this is common with FMICs or just the ARM but be aware. I had to break mine off as folded over didn't allow enough clearance to line up the mounting brackets up top, but as always YMMV.
Also note myself and a few others have noticed that the relocation of the air sensor has the reported temps on the Cluster and the Radio (when not using phone connection) reading a bit high. The mounting bar puts the new location for the sensor behind the crash bar. Prior with a hot engine/sun parking it'll be off by a bit as the OEM location would, takes about 5-10 minutes before it'll report within the 3-5 degree variance. I've so far measured it from the dash cluster vs what my phone reports to judge the 3-5 degree variance. No issues once it's dropped within variance with drive thrus or stop and go and it spiking on my end so far, we'll see as summer heats up.
Went with stealth black for my finish and noticed two minor issues with the finish. One on the back where a small 1/2 inch section of the fins either chipped/scraped or didn't take the finish and one on the front where the finish was bubbled like some dust or other got in. Both cosmetic and not a big enough issue to send it back IMO, you'd have to be about a foot away from the grille or closer to even notice the bumps.
You also -may- have some issues with the OEM rubber/plastic air fins/guides that run either side of the OEM air cooled stack. I had to remove mine due to no room between the FMIC and the hoses for the AC condenser. You may also see slightly higher oil temps - I installed as things are getting warmer, but I do see 230-235ish temps where I would normally see 228-230ish prior; again it's warming up here so take with a grain a salt. I commonly have seen 228ish on the high side if not 234ish when romping with temps during the California "winter" being 55-68ish ambient out. Highest with a romp for oil has been 237 so far that i've seen with it being 73ish outside.
I can say I've been in contact with their customer service since my purchase of their FMIC and DP, and things have been great. Learned the Golf/GSW is 60mm and not 65 like the GTI exhaust with their DP, but they've been great with response and attempts to help sort things out. Had a clamp strip in tightening with the FMIC and they've shipped a replacement at no charge. Once that arrives and I can swap the OEM worm i'll probably run some logs. Their response time has been at longest 3 days via email, and they are happy to help with any concerns I've had.
So far it's been a great company and a good experience all around.
Some existing logs from another source I came across when looking up info:
http://mygolfmk7.com/2020/10/arm-motorsports-fmic-bench-cooling-test/
http://mygolfmk7.com/2020/10/arm-fmic-street-data/
Honestly if you don't want to go over 350-400ish HP, i'd say it's a great buy for the cost over existing kits.