when you un-abbreviate it, yes... it's redundant. but to me, the MP5 moniker has become the equivalent in a lot of ways as the WRX moniker. Similarly, the MSP fits the bill as well. In other words, people see MP5 and they internalize it as being what it is... as opposed to reading it out. Sort've like a chinese character or something.
Anyway... I can't believe no one had that! It's not like the P5 is a new car!