J Tiberius

Now, after 20 years of driving a TT 225 in both hot and cold places I can say the following:
DSMICs >> FMICs ( >> SMIC )

Cyprus has hot. dry summers (40C/100F) and very moderate winters (15C/60F on a cold day). I have two tests: 'highway pull" and "mountain rat-race".
"Highway Pull" is cruising at ~50 and then opening the taps until you lose your courage.
"Mountain rat-race" is a continuous wide-open-throttle and hard-on-the-brakes roller coaster. And I don't mean driving slightly spiritedly, I mean you are going for broke on twisty roads about 1.5x - 3x as wide as the car with the occasional 50lb rock lying in the middle of a corner just to keep you on your toes.

FMIC... a good FMIC, lets say a $600-700 kit (now that it's 2020, prices went down a lot) gets you a really decent FMIC. Add some $$$ for install and pipes/fittings.
When you drive around normally and do a random pulls...it feels better overall. When you look at cold, hard data, the differences are less than you'd expect, but they are actually there.
When you start to drive hard, the FMIC solution starts to show its weaknesses. Mostly you notice the higher coolant temps. You can work around this with a better radiator, but overall, the design of the TT will just mean that you are on borrowed time here. There is no escape from the fact that if you gun it for 10-15 minutes, you are going to hit the red-zone. You might say "don't gun it for 10-15 minutes" but, frankly, if you have DSMICs, you can gun it until the gas tank is empty and the needle doesn't generally stray too far from ~90C/200F
FMIC and max power on a dyno: Yes. There is more power. This is primarily down to better flow, but I suppose the exact setup will play a role here. Most dyno pulls are not done when the engine is hot/saturated. But when you drive hard... this is where your engine is operating.

DSMIC... a good DSMIC setup is definitely costlier. A body-kit can then help put more air into the SMICs. That in turn DOES increase your drag. At low speeds no one cares or notices. Anyone doing this also cares not one bit about fuel economy unless they are on acid. But as you hit higher speeds, the drag on your car does actually affect not only acceleration but stability. In particular the back of the TTs is a clusterf.ck... if you increase turbulence at the front as well, then it really does do unpleasant things. Very critical with a good DSMIC setup is the rear exhaust setup. If you straight pipe it like a redneck, then you have a huge cavity under the rear of an already front-heavy car. As you hit 100-120mph, the air will buffet under the rear of the car and the slightest aberration on the road surface will send you sideways without much of a warning. Even with a rear spoiler. Been there. Done that. Don't want to buy that f...ing T-shirt again. ESP won't do squat at that point but it is a great way to lose weight and a wonderful excuse to buy a new pair of trousers. But on my mountain road rages, there are no coolant temperature issues and there is a nice, clean, progressive power delivery. If you want to push your TT hard on a twisted road/circuit, DSMICs all the way.