I'm still not convinced. I guess my newbie colors are showing as far as GMing goes, but I'm not tied down to all of the calculations that so many seem to cling to so desperately. This system is far more forgiving when you try to relax a little and allow yourself to bend the rules from time to time. My PCs aren't anywhere near 600 XP yet, but if I found they were blazing through things I would simply find ways to make it more difficult without making it unfair or taking away any of their fun.
I know you're pretty set on using your modifications, and that's fine. It's your table, it's your game, you do what you like and I hope it works the way you like. Everyone has their own playstyle, and as long as you and your PCs are having a good time, that's the important thing.