I have played all major GTs with a pad (GT4 also with a FFB wheel). I have also played FM2, FM3 and FM4, each one both with both a pad and a FFB wheel and I gotta say that your arguments are invalid. You can CLEARLY feel the difference when driving a small car, for example Lotus Elise when compared to a old 70s muscle car. Also, the handling is predictable, random losses of control belong to TDU2. The handling feels logical, clutch is better than in GT5 and the difference between the two series are NOWHERE near as enormous as you're trying to claim. Sure, GT5 is a bit more real but if you really want realism, you should plug your toy into your PC and try rFactor, pCARS etc.
*waits for "but GT5 has 1000 cars"*
E: Regarding the graphics etc; the graphics are obviously worse as it's running on an older console, duhh. The green hell has always been wrong in Forza, due to them reusing stuff. That's why it has kerbs that are a meter high.