APR stage 3 injectors are modified S3 injectors done in house by their engineers and flow tested with their equipment.
I understand you don't want to run APR's 100 octane file but it would be considerably easier to go that route. The car was recalibrated to a larger MAF that could have a different voltage output than the regular sized MAF. The low pressure system is stock and then modified electronically by APR's engineers to achieve the correct amount of output. It was painstakingly done by APR in collaboration with VWoA to have a perfect tune that allows the correct advance, EGT's and boost to a point that doesn't go past the OEM MAP sensors capabilities.
Here are the obstacles:
- Different sized MAF
- Different calibration for the low pressure fueling system
- Optimized not to surpass 23psi
- Injectors that are unique with characteristics only APR knows about
If the tuning system you want to use is based off of OEM MAF, RS4 injectors and a mechanical means of raising the LPFP's output it's not going to appreciate a lot of the features that come with APR's stage 3. The people who buy this kit are the set it and forget it type of folks who appreciate the system as is. You're taking a system that has hours upon hours of R&D with unique propietary parts and adjustingment, I don't know what will happen. FWIW, the 100 octane file is no slouch and an Aquamist WMI system will best the regular 100 octane output as it achieves greater cooling and higher than 100 octane knock resistance with the methanol.
To make life easier? Sell the stage 3 kit and you'll have the money to go BT via CTS Turbo/ATP/whomever and grab RS4 injectors, USP Motorsports LPFP solution and run it as hard as you desire. APR's stage 3 was never designed to be altered by anybody but themselves and I think you'll run into too many obstacles and frankly not make anymore power than you would have running 100 octane with Aquamist WMI. I'm sure Eurodyne's solution can recognize, all of the variable that will be unique to APR (in short, custom fueling and MAF readings) but you'll be in uncharted areas as most don't go this route. There's plenty of experts on this forum who run RS4, flapper less intake manifolds, upgraded LPFP and other BT modifications but they won't be as much help with your current set up... Only APR will be capable of helping at an expert level and I'm sure they aren't going to want the liability of what you want to do.