I don't think so. The game is what it is when it's released.
The major problem with games that take a long time is that they tend to be outdated when they release, because the fundamentals of the game were designed in a time that was different to the one that they find themselves selling in. Some of the TLG mechanics were borderline acceptable on PS2. Having a shonky control scheme is death for a game on PS4. Games that have really protracted development times tend to be their own worst enemy, and don't tend to need any help shooting themselves in the foot.
If GTS is a great game on release, the development time doesn't take any of that away. It's annoying, more so because of the tease routine that Polyphony put us through, but a good game is a good game regardless of how it came about. A review is judging what a game is like to play, not the efficiency of the developers production methods.
Having multiple Forza games out might affect your choice of which console to buy, but again it has nothing to do with how good GTS is. It will be compared to the latest games that are out at the same time, and it doesn't matter if that's FM7 or FM10.