An aftermarket boost solenoid WILL void engine warranties. If you blow the motor they will system check your car to look for over boosting in the systems memory.Yeah the MAC is just a better part for additions of better parts. So yeah i highly recommend it.
If you get a quality BOV you shouldn't have an issue. I've used GFB in the past and never had an issue. I have a GFB Respons on my Gt running 50/50 on a high piston load. Boomba are very cheap bov and I would never put one on my car. I've known several people with focus st that had issues with them and switched to other brands. Also the injen intake even not closed still sucks cold air from the wheel well. My IAT was slightly higher by 3° with my injen cai. But flows better. The afe intakes are just a factory intake they put a round filter on. If you want that just buy a high flow flat filter and save yourself 200 bucks.