I think I'd like to remind everyone of something I was told in this very thread, though I can't remember who said it to me. When you have a given car that can take an engine, and the range of possible HP doesn't really differ much between the swapped engine and the stock engine when both are fully-tuned - such as in the DeTomaso Pantera, which can take the Coyote 5.0L - there is still a benefit that I (and maybe others) have overlooked. It's how much torque you get at a given RPM, which can have notable improvements over simply tuning the stock engine.
I believe a notable example of this was when there was a Daily Race that limited players to the BMW M3 E46, and you could tune it within a certain power and weight limit. So in theory, all the cars should've been on equal ground, but I think the M3s with the Z4 GT3's engine had better torque, giving them a bit of an advantage.
I think the best way to illustrate this, is to examine the dyno graph (you can view it when tinkering with the full-customize ECU or the power restrictor) on two cars of the same model with the same HP and ideally the same parts installed, but with one model being swapped and the other having its stock engine. The aforementioned DeTomaso Pantera and BMW M3 E46 are good examples in my experience, but other cars may also be good for this experiment as well, such as the Ford Mustang Mach 1.
So yeah, there may be obvious benefits when you put something like a K20 in the Honda Beat or the Escudo engine into the Mitsubishi Lancer Evo X, but it would seem there are other benefits besides just having a higher maximum HP.
