What we have here is empirical evidence vs theoretical or faith based evidence as far as what one system can or can not do, for Gunstar Heroes or Alien Soldier and the arcade card, well, there are no Treasure developed platform shooters on the pc engine, we only have system specs to go by, but we have direct evidence of how ACD games such as Art of Fighting, Fatal Fury 2, Fatal Fury Special, (World heroes2/World heroes) Strider and Maduo Monogatari turned out on each system.
As for Sapphire on the sega cd, again if we look at all the missing detail and animation of Art of Fighting(sega cd), i would say no.
With all the Mode 7 and transparencies, a direct port of Super Castlevania IV could not be done on the pc engine, but what about a remake, well, who knows, but we do know how a remake of Dracula X would turn out on the snes, going back to the Gunstar Heroes/Alien Soldier question, its interesting to note that Rondo when using Maria is a much faster game than SCIV, Bloodlines or Dracula X.
All said and done, we also have to take into consideration the competence of the developer, the experience with a particular system and how much effort is put into it.