A Polini should not be installed without adding the upper transfer ports. To add the ports, you need to split your case. When you have the case split you should replace your bearings and seals. Since you have to pull the bearings and seals, you can either go two routes.
1. Replace bearings and seals on the stock crank
2. Install new bearings and seals on a new crank and just swap in
I've run a polini with a bushing crank and a performance crank. I prefer the performance crank. With a 15bing, you should be fine as long as there is no play in the bushing.