i had a straight through exhaust with only a small cherry bomb silencer on my series 3 that had a defender 2.5n/a engine and to be honest it made no difference to power/speed/mpg just made it really loud like my tractor.
only turbo landy engines a full straight through exhaust makes a difference to.
made it, cut the old one after the first bend on the down pipe, flaired the pipe, gas welded in a 2" to 3" adapter, then got a 3mtr length of 3" fer £5 , welded it up and used bends where needed, keepin it relitavly straight , easy enough