Andyoxon, your car / history sounds similar to mine. I had Evolve then added AR catted downpipes. I'd say the DP's do make a difference but there are diminishing returns at work here. Namely, going from stock to Evolved is "wow!", then adding DP's plus Evolve stage 2 is "ooh nice". That said, I think my catted DP's make the car sound lovely and growly, not silly loud but nicely aggressive. If you want the peace of mind of warranty, keep stock DP's - as Phil says, swapping DP's in and out is well nigh impossible.
Or do what I've done, forget the warranty, use a good indy, get the car how you want it and enjoy the car.