Domed pistons and flame propagation
 

I heard that domed pistons do not provide a good flame propagation and cause detonation. Is this true? If they are bad, why do they sell them? With flat tops, my CR will be 10.3:1 and I'd like to gate a CR ~ 10.5.

True! Domed pistons do not promote flame travel like a flat top or slight dish piston will. The reason for selling them is it is the best way to increase compression. In your case to go from 10.3:1 compression to 10.5:1 all you have to do is mill the cylinder heads about .014 If you want 12.5:1 compression you will just about have to go to a domed piston to achieve that.
In NASCAR Winston Cup racing the Ford teams went with a Robert Yates designed head because he changed the valve angle in the heads so that the valve was almost straight up in relation to the piston. This was done so that the combustion chamber could be made very small so that the compression could be made without going to a domed piston.