That's good to hear about your experience with an old-ish 300. Sounds like you're having a good time with it. Are you in an area where it would be worthwhile to think about dual-sporting it?
Funny you mentioned riding a WR450 to realize that you needed a lighter bike. I recently bought an 08 WR450 that's dual-sported (plated) and have been having a great time on it. I'm not certain about my gearing, I think it's stock 13/50. It came with a spare 14, so I'm thinking the one on it is the stock 13 (haven't counted the teeth yet). It seems to lug pretty well, but I rrecently compared it back to back on the same trails against my XT225 and was really surprised at how much lower geared the XT225 was in 1st. I was able to creep on it where the WR450 had to go faster or need clutching. The 225 isn't fast & powerful like the 450, but the 6 speed surre lets it lug great. I've never mounted knobbies on it, but I'm sure I'd be amazed with them. I would consider gearing the WR down more, but I don't want to loose the top end for the road.