Compression ratio from hell

[qaawale]

I am getting ready to rebuild an engine that is going to be a template for future attempts and would like to utilize water injection using separate computer controlled drivers for each injector (four for fuel, four for water, eight injectors total).

The engine is a BMW M10 four cylinder, OHC inline design and I can get custom pistons in any compression ratio I want. Using water injection, what's the likelyhood that I'll be able to use a 12.5:1 or 13:1 cr while running premium unleaded? Thanks!