I can't speak to the 6.9 or 7.3L but what rhk said above was generally true withthe 6.2L GM I built up years ago. I installed turbo heads onto an NA engine and the combustion chamber volume was increased. I actually measured it but it was obvious by comparing the prechambers. I'm not going digging for that old data but IIRC it was about a half a ratio (i.e. 20.5 to 20:1) on the GM. The only difference I noticed is that the engine developed a little more boost at the same pump and turbo settings (I had done the pump and turbo prior to the head work), I suppose due to the increased combustion chamber volume. I was warned that a ratio drop might make cold starting harder but there was no change I could notice with a half-ratio drop (it was equally poor either way... my 6.2 was an appallingly bad cold starter). The extra boost (about 2 psi) did not deliver any noticeable power either. I never towed with that rig (it was a diesel Blazer) but I think it delivered slightly lower EGTs at the existing pump setting. Again, I cannot relate actual 6.9/7.3L info, just a some generalities from a similar engine. If it were me, I'd use the 7.3L cups. Were the 7.3L IDI turbo cups any different? Don't think so.
