Can check on hpisavageforum.com, it's a sister site to this one. I know some have used a novarassi 28 as well, which should be a ripper.
I've never run an HPI engine in a savage. Always got them used and put my own in. So I can't compare really. I got a LRP30S2 in my X at the moment. It does pretty good with the HPI polished pipe. I was trying to run it with an LST2 pipe, but it seems to not have liked it. Once I changed over to the more free flowing HPI pipe, it runs quite a bit better with my limited tuning ability.