Supersprint told me the standard system was about 78db, their non racing version was 85db and they didnt have figures for the racing. However I was measured at 93db static test at bedford - that was standard manifold and pre-cat, standard cat, supersprint racing centre (non-resontated), supersrpint racing back box.
Decats usually increase by 2-3db on a car like ours (think that's what Scorpion quote), so I would expect your current setup to be about 92-95db - based on the above.
Easiest thing to do would be put the cat back in and see, then swap the centre if still too loud - might find someone on here to do a swap?