JE makes an inverted dome (essentially the same "D" shape), but again the dish is -39 cc's. I believe Diamonds are -36 cc's and have a .005 lower compression height on their stroker pistons (1.345" on Diamond vs 1.35 on JE). The stocker piston dish on both is very big in my opinion and leaves you with little choice other than a custom piston to get around 10 or 11:1 on most popular stroker combinations (4.3 & 4.5 stroke, 72 to 80 cc heads). The flat tops leave you around 12.5 or up.
Either will do custom dishes. In my opinion you can't go wrong with either of them. I know there are many custom pistons available, but I'm not aware of any other "D" dish off the shelf forged pistons, I'm sure if there are someone will chime in.
If I recall correctly one of the piston makers, I am thinking J&E but could be wrong list thier flat top as 12cc valve reliefs. You asked for 15cc dish well thats only 3 cc difference and no issues with the quench due to a open dish. I am thinking you could take up that extra 3cc by either cutting the valve reliefs slightly larger or some porting, polishing work on the combustion chamber pretty easy.
let me ask one other thing then. if the only shelf pistons in the flavor i'm after have 25cc dish which is for stock stroke stock rod would there be any possible to mill .2" off the piston and use a chevy 6.8 rod? if it would be possible what kind of dish would that leave me with? i haven't heard the difference in wrist pin sizes so i don't know if it works surely someone here does though