As mentioned already there are also options with regard to what level of stage 1 map you’d like. Most good Tuners will know that the default map to offer on the M157 is between 650-680bhp. This is mainly driven by the fact that in the early days of remapping the M157, it was incredibly easy to go for over 750bhp and 900lb/ft. However, a couple of years afterwards there were stories appearing about engine failures etc, which could have been caused by the remap (e.g. too much boost low in the Rev range = high temps). On this basis the map was toned down. This doesn’t mean to say an M157 can’t be remapped to 700 plus safely, as it can, but it needs a lot more work to check that knock, fuel pressure, timing, fueling and temps are all optimal in order to create a safe map.
For the record MSL / BFT etc were the first to change their maps as a precaution, however due to the quality of their map writing I don’t think they had issues with the earlier maps, it was mainly Tuners who “sold” pre written bought in maps.
With the above and then all the other variables mentioned, e.g. plugs, filters, fuel, oil and not forgetting HP fuel pump etc, then it’s easy to see why the results aren’t the same every time. If I’m honest then I am always sceptical when a tuner posts numbers on a certain turbo engine and they are almost identical every time. On NA engines yes this is possible.
Going to SRR is a good idea. But make sure you go back post remap. The dyno dynamics dynamometer that Charlie uses is different to the one at MSL, so it’s likely the readings will be different. A Dyno is only a guide.
Hope this helps
on mapping though 
