FP is a good class for EESCC, your have pretty light competition.
raw results from previous event http://www.eescc.org/Publications/2010Event4Results.pdf
i'm there under Street Modified in 6th (first time ever doing autox in this car, previously i had a 95 impreza FWD mostly stock in ST/STS)
and nick placed 5th in his impreza (he lives in eugene "rs_to_wrx_swap" here on the forum)
XP can get dangerous because there are some very capable cars in that class since its kindof the anything goes category. SM has kinda been the go-to class for engine swaps (subaru) although there are a lot of guys in that class that arent legally in it.
Yeah I was about 6 tenths off of one of the faster XP drivers so I think it is possible but for now I plan on FP. SM is fun there are a lot of subarus but like you said almost none of them are SM legal.