i have a '95 wrx sti totally standard apart from 2.5" s/s exhaust from standard down pipe. i do have a de cat down pipe but 02 sensor hole is top on de cat and standard is down bottom can i use this with no mods? also what other basic mods can i do to get some good bhp results on a reasonable budget?

 

wots the budget

 

at the moment about a grand