"1500" has always been the grit level of the sanded ball before the polish is applied. You actually have 2 concerns here.

1. There are 2 grades of grit or sanding level. FEPA is the European grade now used by about 90-95% of all abrasive manufacturers (except Japan, I believe). CAMI is the older American standard or grading level.  FEPA grades are usually preceded by the letter "P"; here on ballreviews.com and many bowling sites, the P is hardly used. Most sandpaper manufactured in the United States  used the CAMI grading standard. Today even they use the FEPA standard.

1b. P1500 grit (FEPA grade) is equivalent approx. to 750 grit CAMI grade.
P3000 grit FEPA is roughly equivalent to 1500 grit CAMI grade.

2. Polish can very abrasive, like Brunswick's Rough Buff, Storm's Step 2,  or Brunswick's Factory Finish polish or Storm's Xtra Shine or hardly abrasive (such that it's consider non-abrasive) like Valentino's Snake Oil and Lanemasters Factory Finish.

2a. If you apply an abrasive polish over a P1500 grit sanded ball, and then the resulting surface will closer to P3500 - P5000 with a high gloss shine.

2b. If you apply a non-abrasive polish over a P1500 grit sanded ball, then the resulting surface will be closer to P2000 - P3000 grit + a high gloss shine.

2c. The result of applying any polish will be determined by both the underlying starting grit level AND
the amount of polish applied, the amount of pressure applying it, and the amount of time on the spinner it is applied.

He knowes surface like nobodies business.

Thanks, Guys.

Good luck, TheGom; hope it helps you. Playing with surfaces takes a little time and a little experience, but if you're rev dominant, like I am, it's almost essential to learn what small differences can mean to your game.