standard bore is 2" i believe, my DP is 3" i imagine the miltek will be similar. With a reducer i think u should get away with it, but you'll get better gains with a larger bore centre section and back
 
I have a miltek down pipe and its 2.5" and its bolted onto a supersprint cat back exhaust system which is 2". Its has a reducer to join them and i have had no problems with it so far.
 
cool. at present its only got a powerflow backbox on it (how i bought it) but its only temporary until i get a milltek cat back too.
cheers guys