But how much better realistically is it going to be over revo day to day?
As Lee has said, finding someone who has had both will be your best source of information.
One thing worth noting from doing a lot of research on here and I mean hours of sieving through threads, is that Revo stage 2+ gains are the smallest across the board. Dyno's are obviously a bit of a lottery, however several S3 owners have reported only seeing 330bhp at best from Revo's stage 2+ software with all the required hardware, which is poor considering they charge premium prices. APR stage 2+ customers are all within the 370bhp region which is what you should be getting at this stage, the main reason Revo's software is inferior nowadays is because they haven't updated their maps for years whereas APR have (V3.4). Custom maps with a reputable tuner are still the best way to go though as you get the most out of your car, so even though R-Tech is a bit of a drive and more expensive, you will get better gains.
I'm currently Revo stage 1 with a Revo CAI (Camcoated), not so fussed about the figures at this stage as it's more about the drive-ability which has been improved due to the increased torque. However, after reading comments from several disappointed Revo stage 2+ owners, this has definitely turned me off using them again in future. Even though it is my cheapest option and budget is a huge limitation for me, I will now be using Storm Developments (S3Alex recommended) instead for custom mapping. I'm really happy with the Revo CAI as it's a great piece of kit but I'm kind of regretting paying £600 now for a Revo stage 1 remap when I could've just gone to Storm Devs for £250 instead, no doubt they offer discount for continuing to use them when progressing through stages as well.