Dont go for an aftermarket CAT - they are all ****!
I had a 'decent' aftermarket CAT with a 2yr guarantee (about the price you mention, so probably the same company, PM me to confirm) and it wouldnt go through the MOT after 11 months on the car - simply wasnt big enough and was full of cheap metals rather than the expensive stuff. Was a total nightmare clamining under the guarantee but I did manage in the end.
Wait until it fails and then get a known good OE CAT from a breaker. I paid 50quid for a 2nd hand one and its perfect.
REPEAT - DO NOT BUY CHEAP AFTERMARKET CATS - complete false economy.
-1998 916 Spider 2.0TS Lusso-
H&S Corse stage 1.
CATCAMs stage 1 cams with Savali custom ECU remap.
305mm discs with 166 4-pot callipers.
Ragazzon exhaust mid section.
Biscione Racing 2x70mm rear section.
17" Zender Siena wheels with 3mm rear spacers.
etc etc etc!