My RX8 compression is 6 and 7.4
I think it is too low, need to rebuild.

But if I swap to new starter (2Kw), can mask the long hot starting problem?

 

Now my rx8 idle is stable and nothing problem when I drive the car

Just is long hot start

of course, when I race with other good condition rx8 I will lose><

 

If you have failing compression, test, a faster starter than you have will mask the hot start problem for a while. It certainly won't mask it forever, and you can't just keep upgrading starters to try to keep masking it. Find out what your current cranking RPM is, and you have to beat that to see an improvement. Once you have hot start problems at 310rpm though, there isn't anything higher you can go to so you are SOL.