Finer grit under the polish or polish on top of polish will only make it go longer and snap harder at the back and make it more susceptible to carrydown or spotty conditions.
Sanding it to 3000/4000 will usually give you slightly less length, slightly smoother backend and more control. Under normal circumstances, 4000 grit dull is equivalent to 1500 grit polished in oil handling.
Sanding it to 2000 grit or lower will add more control with even earlier hook and still smoother backend (on the same oil amount and pattern) BUT will increase the amount of hook and WILL REQUIRE more oil.
An alternative is to use a slip agent polish. Slip agent contain chemicals to help clog the pores or resin balls, reducing oil absorption and reducing the amount of hook, increasing length and REDUCING backend. (The rate of oil absorption is one of the major factors that influence the strength of hook for resin balls; so clogging the pores reduces hook.)
