Comparing a 2-wheel drive to a 4-wheel drive in my opinion is like comparing oranges with apples. I assure you my HPI MT 1 ¾ is far from slow with the OS 18 TZ in it. Most RTR come with a sports engine and are not real fast. Yes I think the Firestorm will be faster because it does not have all the friction of 4-heel drive. Now if you where to run a stock Firestorm against my MT I do not think it would have a chance.
As far as putting an Axel 28 in the MT even with a SS you better get ready to do other up grades. My MT 1 ¾ with all SS drive train when I put the OS 18 TZ in I still had to make changes to keep the drive train from breaking. The OS 18 TZ is a drop in engine and has as much HP as many big blocks and has more power then you will ever need. But if you just want a project go for it but I would not do it if it is your daily runner or only truck. Matter of fact if it is your only and daily runner I would not put the OS 18 TZ in it. [X(]