According to Tony William's site, the Hispano had a heavier round, moving at a higher MV. It should be no wonder that it's a significantly better gun than the 151/20. I mean, each hispano round is 28mm longer than the 151/20 round.
As a Yak Flyer, I think the differences are simliar between the ShVAK and Hispano, with the ShVAK round being 11 mm shorter. Although I notice the difference more in range, than in hitting power (my aim is for ****, and I have to close in a LOT closer to hit with the ShVAK, than with a Hizooka, or .50). I imagine this has something to do with MV, but I really don't know.
When it comes to the Elephant guns though, I've never noticed the 30mm to be at all underpowered. Especially if you consider how many rounds you are hitting with. Compare the single 30mm on a Ta-152, or a 109G, to the 4 Hispanos on the CHOG and Tampoon, and I'm sure you can see the difference in weight of fire alone.
-Sik