Remember the 75mm was designed from the beginning to be an infantry support cannon with HE shells.  The AP round came after.  The 76mm was just the opposite.  Keep in mind that the design of the warhead is more important than velocity when speaking of HE shells.
I tested each HE round for damage and found the following results (vs a hanger; # of shots to = 2871lbs dmg):
Tank Main Gun Statistics for Aces High
Country           Tank             Caliber/Gun         Reload Time                   HE Dmg (in lbs)
US         M4A3             75mm M3 L/40         3.6 sec         173.8
US         M4A3 (W)      76mm M1A1 L/53      3.6 sec         103
US          M8        37mm M6 L/53         2.1 sec         31.2
US         LVT-4     75mm M2 Howitzer      3.6 sec         156
Britain      Firefly     Q.F. 17 lb Mk. IV L/55           7.2 sec         146.3
Soviets      T34/76     76mm F-34 L/42              8.2 sec         156.2
Soviets      T34/85     85mm ZiS-S-53 L/52           6.8 sec         231.8
Germany           Pzr IV             75mm KwK40 L/48      5.4 sec         156
Germany           Tiger I     88mm KwK36 L/56      6.3 sec         234.3
Compiled by “BudGray”, Jun 2010
The 76mm is lags behind the 75mm by almost %70 in HE damage.  Point being... dont take the 76mm to pound a town.  
 
   As you can see the Tiger and T34/85mm are very close at 230lbs damage, and the US M3 L/40 cannon on our M4A3 Sherman is only second to those 2.  TRUMP CARD PLAYED.... factor in the reload time and ammo load (heck, even throw in the rockets, too) and the wee little 75mm cannon is barely behind the Tiger and in front of the T34/85mm in the amount of damage over time in one sitting.  
Last, but not least, I emailed the above info to the AH trainers to have posted but I never got a reply.  
