"[Sony] doesn`t support PD the way they deserve"
Goodness me
Sony have done nothing but support Polyphony Digital. Sony gave PD plenty of time to craft GT5, together with a nice budget. They sanctioned an expansion, and seemingly a hiring drive. PD are also likely to be the first studio anywhere in the world to get their hands on PS4 tech.
If you're basing your reasoning on marketing, firstly, Sony are ****ing awful at marketing, and second, other titles probably need much more promotion than GT.
You also have to remember that while significantly increasing the headcount to that of Turn 10 levels would be nice for gamers, it decreases the margin for Sony, as the cost of making the game dramatically increases.