I don't really understand why you want to increase the rev limit, when you are talking about a Cupra nearing 400 bhp. What optimised torque are you talking about? A boost 9 Revo'd with full extras (exhaust, intake, fuel pump, dv, intercooler etc) can produce 50+ kilograms of torque at about 4500rpm - do you need more? What should be a concern is
how to put the power down on the road and not
how to produce more. I am talking based on my experience and not theoretically. I have a powerful Cupra with all of the above for everyday driving and I've driven mine with maxed out settings and the best word to describe it is 'scary'. There is no need to reach the rev limit to reach high speeds - you can do it on 6k rpms and there is plenty of power in all the rev range. Unless you are into street racing you shouldn't bother too much with pushing the rev limit. Regarding the 406 bhp you've seen it is the corrected power which is a bit optimistic I might add but again they used methanol so it's not realistic. You could change injectors to push it some more but you need a custom map. In any case you'll be maxing out your car's performance and I don't know if your car can handle that for a long time. If you want more a move to big turbo would be wiser and of course change all the appurtenances that might be necessary.