Hmm.... a topic that's near-and-dear to my heart... cars that need better engines!
The Mazda Miata NC doesn't really
need much more engine, but a turbo-charged version would be nice.
The Mazdas that really need more power are the RX8 (already mentioned) and the Mazda Protege.
The Protege was considered by some to be like a front-wheel drive BMW 3-series... it had good balance, great steering feel (better than the Mazda3 that replaced it) and terrific driving manners. Unfortunately, Mazda's 1.8 and 2 liter engines at the time were the FP and FS series engines. The FP was decent, as far as 1.8s go, but the FSDE was pure crap. It was torquey, but had little room for performance improvements, poor head-flow, was asthmatic at high rpms and wasn't a very durable base for tuning. The penultimate version of the Protege, the Mazdaspeed Protege, was praised highly for its handling, but suffered from too little power... 170 bhp from a
turbocharged 2 liter engine? Honda was getting 200 bhp without a turbo at the time... and even the Japanese market Familia version was available with a 170 bhp naturally-aspirated mill... surely Mazda could have done better!
Another car that deserved a more performance-oriented motor is the previous generation Nissan Sentra SE-R:
Yes, the QR25DE was a torquey mother... but c'mon... a car like this deserves a high-revving screamer of an engine... torque may make for better day-to-day driving, but the QR was never as lively a mill as the SR20 it replaced... an engine that could rev over 7000 rpm and was good for over 200 bhp (even without a turbo) in naturally-aspirated form. Big mills may make for more performance, as their average horsepower actually make for better acceleration, but sometimes, character is more important than outright power.