In fact, its the RP portion that I think is more important to an RPG than the G.
Well, that makes sense, because it is the RP portion that differentiates RPGs from other Gs. If youaren't going to put emphasis on the RP, you might as well play some other G with less overhead.
You can have almost no game mechanics, or extensive simulation attempts, and it doesnt effect my perception of an RPG.
So, there is a point where the G takes up so much of the cognitive effort of play that the players are thoroughly distracted from the RP. Exactly where that point lies depends on the player, the rules, and the tools at hand to support the player.
Rather notably, the STYLE of rules and player matchup can be more important than the AMOUNT of rules. Different players can internalize different types of rules differently, with the end result that, say, while D&D 3e may be an overwhelming mess to one player, that same player might not have much problem with World of Darkness rules.
So, really, your mileage may vary, and we should not mistake our own driving for the only way to drive.