DCOE 28s? Never heard of ones that small. We normally run DCOE 40s on the A-series.

A15 uses A14 cylinder heads. No difference. There are several different design A14/A15 heads though. Which one do you have?

Skimming the head to go from 9:1 to 10:1 will give about 3% more power. That is 2hp.

Yes, if you want to rev the engine higher than 6000 RPM, get double-valve springs. For that you will need DCOE 40s, and a better camshaft. Better to swap in the entire A15, and use a GX camshaft (256/256 degrees). Most A14 and some A15s use 248/256. Later A15 uses a very mild cam.