Paying a pro is well worth the $$$ in my opinion.
I have done my own and had decent results by not going overboard.
It is hard to quantify because I also did some suspension changes.....it was an over the winter project that netted .15 on the E.T. , but what helped most is hard to say.
It picked up 3-4 m.p.h. in the eighth though , and that indicated motor over suspension I think.
I have an old Makita electric ( Japan ) and it is a war horse , but as production went over to the Red Menace , quality went south.
Same for Milwaukee , and probably every other tool maker.
If you are going to use it , buy quality.
If it is a one shot , buy junk.