Piercing I think is diminishing return. After a certain point, you are modifying the armor values of a tank into a 7+ and that doesn't do anything more than make it more impossible to make the save.
I mean, If the best Allied armor save is a 3+, then a piercing/5 weapon is not necessary. I'd put a cap on it, to be honest and just increase the points by aroun 30% - 40% each time. I think a 50% increase would be too high, while a 25% increase is too cheap.
Low velocity weapons seem to be D6. High velocity seem to be D10. I don't know that low velocity weapons have piercing, or should, anyway.
Multihit is the one that makes me boggle... like when I tried to stat out the M4(105) Sherman. It has a 105mm howitzer, and that is something that does not exist in the game right now as anything except off board artillery, perhaps. I know a 105 can pop a German tank when firing directly, but there are no conventions for large caliber rounds. Only low or high velocity, and it seems that HE weapons are pretty much not in the tank killing business.
So, I give it a D6+5, like other low velocity HE weapons (mortars and such) have a D6, but give it multihit without piercing. That way, while it CAN damage even a Panther's 9+ frontal armor the tank gets its full save but if it fails.. it is gonna be hurting - crippled from multihit.
On smaller stuff, like a Panzer IV, it has a good chance of scoring a hit, and if it fails the save it WILL be destroyed as it only has 2 hits.