I would let it run for awhile at semi-painful levels and check the fins. The human hand is very sensitive to temperature differences. If the new fins feel almost as warm, or just as warm, I would pronounce them Satisfactory. If they are pulling that output transistor heat away to the edges of the fins, that is the goal and you have achieved it.
I had one of those little 80s Technics receivers with a New Class A amp section that ran like a furnace and my friend thought it was going to catch fire when I gave it to him. Also a Hitachi labeled Class H and both were less than 25-30 watts a channel but I gathered just about the most inefficient amps I ever had that had no tubes.