Xsi horrendously down on power.

[Matt]

Right I have an issue. My xsi is seriously down on power, had a little thing of the lights with a fiesta that had been up my arse for last 4 miles, it was a mk3 si which is a 1.6? Any stupidly standard even standard exhaust etc, and it was by my side, surley they are not that quick?! Or mine is seriously dying.
What could cause my loss of power?

And before anyone says it (fooby) no '6 shit haha.
I believe I have a small blow in the Mani/cat join, also possibly a small hole in the exhaust. It's overdue it's oil change etc. but would this cause it to lag? What about a change of spark plugs? Would that make a difference? I really don't understand why it would be the same power as a 1.6 lol