It's the exact same engine as an Abarth, the tune was the only difference as far as power is concerned. So at 135hp you were already 25hp down against an EPA approved calibration. So that +25 HP is basically "free HP" they left out for marketing/sales reasons. Beyond that, 180 hp is generally the limit in European models, so one could argue that's where emissions become an "issue". So essentially another free +20 hp. That's +45 crank HP basically "free". Everything beyond that is starting to get into "might cause increased engine wear/drivability over time" .. that's where you are required to use 93 octane, change your oil more frequently (suggested max is 5000 miles when tuned), and go a bit easy on the launches so you don't break things. Generally somewhere around 200 to 210 is considered the safest.
The main reason IMHO that the factory doesn't put more than 180hp into these cars though, besides marketing, is the really lose a bit of margin of safety at that level. You start to get fast enough where the short wheelbase is a liability. For example, you talk about how quickly it got to 106. The brakes can't match that speed. If you try to brake hard at 106, the car will get squirrelly and start to swing around on you into a spin. Snap oversteer is a thing. Too much grip and speed in the corners and it can roll. All no issue with "normal sane" levels of driving. But to be brutally honest, if these rolled out of the factory with 250hp under the hood and 2500 lbs on a wheelbase this short - they'd be considered a deathtrap. Experienced drivers can handle them, and they're fun as hell because of this quirkyness, but just like on old 911 Turbo, you wouldn't want to send your teenager out in them.
Another difference on your turbo Porsche is that it didn't have modern computer management. I'm not sure if it had a variable valvetrain either. The Fiat has a fully variable intake valve system. It also uses a wideband O2 by default, and is able to adjust fuel mixture and spark 'on the fly' to avoid detonation which many older turbo systems didn't. Overall just much newer technology.