Firstly, asking the dealer for this was never going to work. Someone would have to actually think and that's not what they do.
You are running too much off the 12v from your description. What you want is an auto electrician to run a fused power wire into the cabin, from there you can opt to...
400hp and 500nm is doable on stock internals. But its hard on the engine, as a tech on these cars I myself would do it, but run high quality oil(motul, redline) and 5000km changes
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