Again it depends on the boat but 8 inches is way deep. I would say normal is about 5 inches.
I think you could go with a -3 safely and worse case space it back down if needed. I would not go with a -2.

The picture depicted is not my boat but thank you. I will take any other input and expieriance as well.

That is really deep. Larger non stepped single V's need the prop deep to carry the bow, but that is really deep. I had my 28 Pantera at 4.75" below with a notched transom. I would go with a -2 since a -3 needs a different upper case or needs to have the regular case custom modified.

It really depends on the boat. With your 28 straight bottom V and the power you have, I would guess you want the propshafts at about 3" below the hull. That is just my semi educated guessimate though.
I would try and compare some similar hulls like a 28 Pantera twins and maybe the 28 AT twins (only 2 or 3 of them made). For reference, my 32 AT with boxes is about 3-3.5" and I'm pretty sure I could go to a -1 lower without issue. I remember somebody with an older 32 on boxes went to -2's and he had to switch to 5 blade props. Worst case if you go -2's is you have spacer them back down part way.

I go -2 because it is much easier to find.Have only 27 feet and straight v hull.
After hear your feedback i think this is a good working option..
With -2 lower propshaft is 5-5,5 inch under hull.
By the way,i use 28 lab Bravo and a Hydromotiv p5-x so not any problem to get on plan with this light boat and 662Hp.
I have new XR upper and this is what i use next season.For savety i buy direct and driveguardian with the blowerengine last year.
Michael

Great information, thank you!