After all those years, hours of tactical gameplay, and having played SHIII, SHIV and SHV. I kept a Falcon Mach V from upgrading to remain on XP and the only active programm on that machine is SHIII GWX + SH3 Commander + mods.
I found the game play, replayability and entertaining level superior for SHIII to any other SHXX despite its 7 years old engine and core flaws.
Moreover, still new mods are created for it. 7 years after it's inception, it is still alive and kicking!...It is a classic as a "1965 29 Riva Super Aquarama" can be!
SHIV and SHV did not add much but better graphics, some different physics, and different machines and theaters of Op., and no revolutionay concepts or improvments over SHIII. Graphics in a Sub can be useless at times. Consider "Dangerous Waters" with its simplistic graphics, the best modern Naval Warfare simulation available to the public (with Harpoon of course), yet their graphics are not what I would call a "First price of Advanced graphics technology and innovation" by today's standards.
Fluctuat Nec Mergitur!