I did the exhaust first, then flash, but not necessarily for any reason in particular. I'd heard good things about Switzer on this board, he is pretty close to me (2 hours), and I had talked to them a bit on this forum. I'm very pleased with the flash, it takes boost levels from .8 to 1.2 bars max and is a HUGE difference. Occasionally, there is some sputtering in fourth gear if you are driving very slow, but its rare and the flash is 100% worth it! Note also you'll not be getting as good of gas mileage. I average about 13mpg now since I'm always on the loud pedal as much as possible.
Switzers flash with cable cost me $1100 I think, and it was worth every penny. The flash adds about 70hp alone and cost 1/2 as much as my used Europipe exhaust I got for a steal at $2200 shipped. The two together add about 85-105hp probably, depending on if you have cats and 70mm exhaust for faster turbo spool and lower backpressure. The growl and turbo spool is also quite nice

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