Engine light has come on because ive got decat downpipe on WRX 02 which is no prob but when accelerating hard it coughs hard fo a split second. Is it because of the light?

 

Could be overboost, try accelerating less Aggressively, and more progressively. works for me.

 

Sounds like overboost to me.
Get your boost solonoid & pipes cleaned out.

 

Nicky, is that the solenoid on the driver side front suspension mount with the pipe running to the 2nd airbox? Just asking as the pipe was not connected properly when i bought my car and i've noticed a slight stutter like the one mentioned (kinda like fuel cut) once or twice in the last few weeks. I've reconnected the pipe from the solenoid and made it airtight (the original connector had been snapped off). Just wondering if it need cleaned out? Whats the best way to clean it? (the solenoid, not the pipe )

 

Marty, If your still getting the stutter under hard acceleration with the pipes all connected then the solonoid & pipes probably need cleaning through.
This can be done with brake cleaner (fiver from Halfords), Just take the pipes off (remembering where the go back on)and flush through. Use the same cleaner on the solonoid. There are a couple of wires under the dash - black in my case - that can be connected up with the ignition on, this will then put the solonoid through a check cycle which will switch the solonoid on & off a few times. Clear out the cleaner, put back together and your sorted.
Use the search facility here for more info if need be.
Good luck.