Upon initial testing, the MR-2, when driven gently, did rather well. The oversteer was controllable and progressive, but omnipresent. However, that is under very, very gentle driving, about 80% maximum pace. At high intensity, the soft suspension hurls all the weight forward under braking, the mid-engine layout allows the car to pivot whenever it (but not necesarily you) wants to. The car ends up at extreme angles, relying only on backwards weight transfer from acceleration to come out of the corner... not sideways. It makes for a frenzied, unsmooth experience unless driven with perfect skill. The only upside, really, is that there is no wheelspin whatsoever, but that only means that you'll have more ability to pay attention to the catastrophic handling. The soft suspension, MR agility, modern grip, and weakish engine all mean that this is a car only appreciable when driven with incredible skill. And I, sir, do not.
The Mustang, driven at the same 80% as the MR-2, feels exactly as you'd expect a car to feel in GT4: predictable and safe, though not necasarily understeering. Driven at full clip, the first difference that becomes apparent is the wheelspin: there is some. Not too much at all, and very controllable. You may need to go 80-95% throttle sometimes, but that's to be expected here, and not a nusiance in the least. The gear ratios seem awkward on the setup screen, but they work to keep wheelspin predictable while driving. At the limit, the suspension feels tight and buttoned down, the car perfectly neutral and feeling (much like the Mugen S2000 of the same tuner) like the rear tires are about to, but aren't losing grip. In other words, it's a perfect balance.
Then, we mash the throttle. The car swings predictably sideways at a light angle, requiring that 80-95% throttle. You know that the wheelspin is the only thing keeping you from snapping back and hitting the outer wall head-on, like most FRs in GT4, but the wheelspin here is so predictable, it just never happens. Exiting the corner, you can end the drift with less throttle or with more countersteer, the car responds well either way. The suspension feels, if anything, just a little too tight for absolutely perfect control: the reflexes are so immediate, you get the sense your tiny driving errors are being magnified into small missteps of the car. When the car does snap back, though, one may simply ease the throttle, and turn back in the direction of the corner as the grippy rear wheels refuse to have the car spin.
In terms of FR versus MR, the FR car wins the drift battle for its slower reflexes allowing for a more forgiving drive. The MR car is too nimble to be driven with the confidence and smooth motion of the FR car in this case. And, since the FR is naturally more oversteery than the MR, it can use more conservative and predictable settings.
In terms of I4 versus V8, the torquey nature of the V8 wins since it delivers oodles of power in a mellow way, perfect for drift control. The slightly weak nature of the I4 here means that drifts can't rely on engine power, which forces the job on chassis tendencies, which are already difficult enough as is.
So, the victory of FR and of V8 both point to the far-superior Mustang as the winner here. It is still a slightly difficult car to drive, since drift on high-grip tires is slightly unnatural, but it does it infinitely better than the MR-2 does. The only way to improve the Mustang that I can see would be to slow the reflexes of the car just a little bit. It just doesn't feel like a muscle car when it's so immediate and responsive. Other than that, it's great.