i have a newage prodrive running 330 and currantly gavering bits for 400 which can be had on standard internals and maintain the reliability
Nice tight reliable power and not to expensive to get more bhp
I only use mine as a toy so fuel not a problem although some say they are thirsty but what isnt when you put your foot down!
Like anything else, if its looked after it will look after you
A p1 is not really something that you want to mod its a ltd future classic. If its a classic shape you are after i would go for a nice v6 wrx sti type r or ra......280 as stock and scope to go more
Good cars whatever you decide to do
I was plesantly suprised with mine

Regards
Ian