I installed 4 guage yesterday, and it took all of 10 minutes to pull the wiper cowl and reinstall it when I was done. It probably would have been even faster if I hadn't been shivering like whoa...
I would definitally say that this is the safest most reversable method of installing the wire.
Getting a drill up behind the firewall will be a royal pain in the ass from either side, and there's wiring and whatnot you don't want to hit on both sides.
