I don't know where you're getting that SC's are in issue on this car. There have been plenty of solid SC installs on it, and GTM can set you up with a 500rwhp SC if you ask them to. You don't get to the same peak HP numbers as easily as a TT setup, but IMHO the power is more usable in a practical sense, and the whole setup's a bit less tricky and overcomplicated. I'd recommend the GTM over the Stillen. There are a number of solid Stillen installs out there, but IMHO it just seems like the satisfaction rate has been higher with GTM's kits in the big picture.
On the 3.7 NA side, I don't think it's struggling to breath as much as you think. There are just practical limits to how much volume you can push through this engine. The Stillen Gen3's seem like plenty of intake for the engine. Combine them with a solid exhaust setup (the practical pinnacle being FI's long-tube headers + catback) and a good dyno tune and you're getting about all you're going to get out of this engine with just bolt-ons. A whole lot of personal experimentation has been done on this engine in stock 3.7 NA form with bolt-ons over the past 3 years by a lot of smart mfgs and home DIY-ers.
It would be surprising if anyone found further significant gains in this area at this point without boring/stroking the block. Once you do that you've got a whole new engine of course, and you might need to play with the heads too, but then you run into the practical problem of whether or not you can do it in a way that's VVEL-compatible, since nobody's had much luck yet actually programmatically altering VVEL's workings with an ECU tuner. By the time you sort out this 4.5L setup it's just going to cost you a lot more than the proven SC/TT routes.
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