I just wanted to jump in and say a few things and then I'll leave you guys alone
Understand that an intercooler is an intercooler is an intercooler. The only difference between a TMIC and a FMIC is where you put it. The placement is important because the surrounding environment will determine how much heat can be rejected or absorbed. That's all we really care about.
The amount of heat that can be expelled is going to be a function of the airflow going across the core. If you can increase the available airflow, then the intercooler will be more efficient. Top mounted intercoolers are obviously restricted by how much air they can flow past them, so they're cooling ability is clearly limited. You can only flow so much air through a n air duct. Now, this doesn't mean that you can't make a TMIC work for a particular application, but you are limited by how much air you can get across the core. A FMIC on the other hand has a nearly limitless airflow potential, since there are no ducts or passages feeding the core. So really, the faster you drive the more efficient the core gets (to a degree!).
As far as heat absorption, if the intercooler is at a lower temperature than its surrounding environment, then its going to absorb heat. That's just a law of thermodynamics. So when you place a TMIC on top of a valve cover, if the TMIC is at a lower temperature than the valve cover, then it'll absorb heat from the available conduction and radiation. And, as that temperature difference increases, so does the heat transfer from one body to the other. But this also doesn't account for all of the other waste heat emitting bodies in an engine bay like the oil cooler, the turbo, the radiator, etc.
On the other hand, a FMIC has almost no heat input because the only warm body near it is the radiator. Since the radiator is behind the FMIC, and cars drive forward most of the time, the only mode of heat transfer going to the FMIC is going to be by radiation, which is consequently the weakest form of heat transfer. Of course when you stop there may be some convection, but the amount will be minimal because the core is in front of the radiator, as opposed to being on top of it.
And in regards to overheating a vehicle with a FMIC, that should only happen to poorly designed kits that use excessively large intercoolers. Also important, is the distance between the rear of the FMIC and the front of the radiator. If the FMIC is pressed against the radiator, then sure, virtually no air will pass through the mess of heat exchanger fins. But our FMIC for example is pulled away from the radiator so that if the car does get hot, the fans will turn on and air will be sucked in between the FMIC and the radiator. And from all of our testing and beating on our SPEED6 test car, we've never seen any evidence of overheating. None. So if you do your homework when looking to purchase an FMIC, there really shouldn't be any considerable drawbacks as compared to running a TMIC. There are many FMIC kits out there (for various vehicles) that are very well designed and have no consequences when changing fron a top mounted core to a front mounted one.
Although my argument is obviously slanted towards running FMIC's, TMIC's have their place too. They're cheaper, easier to install, and they help keep a sleeper profile if that's what you're after. So I don't think anyone can say a TMIC or FMIC is better than the other. It really depends on where they're placed, and how well the kits are designed since you can make both perform very well in specific applications.
I'm doing very well with resisting the credit card and building the short-term savings account for now...
