I researched the pre-cat delete before going stage 1 in June and believe the consensus of the benefits are +3-5mpg, +5-10hp and +10-15lbft after the stage 1 map, compared to a bone stock car mapped in the same way. This is certainly the case when using someone like r-tech where the map is customized to the hardware; a generic map probably wouldn't take advantage of this simple mod, i.e. some fettling would be required if the car wasnt originally mapped for the delete. Niki at R-tech commented on the torque benefits especially, manifesting in 322lbft for me
Personally I went with stg 1 R-tech where the only "mod" was the pre-cat. Stock air box, stock air filter, revG DV, new PCV, fresh service. The result - 250HP & 322lbft - with the power over a nice wide range. I honestly thing bang-for-buck this was the best setup. Just my two penneth!
p.s. pudding - I thought most, if not all, stage 1 maps wont take account for anything other than the stock main-cat and so you risk EML and poor air/fuel ratios (rich/lean running) if stg1 with a down pipe?