i've not heard of people doing 6 AS-levels! i went to a very good college, and they tried to tell me i wasn't allowed to do 5. not that i lsitened to that.
you sound like you're certainly capable of doing those!
i did A-levels: Physics, MAths, Geography, and i was fine, managed to get AAA
(no A*s back then ;P)
if you REALLY want the highest grades, it may be better to maybe just take 3? but if you personally think you can handle 4, then go for it! geography isn't particularly taxing lol, and heard economics isn't too bad, but i wouldn't know haha, if you think you're up for it, go for it!!
ps. A level, did 5, Physics, Geography, Maths, Environmental Science, and French. there weren't enough periods in the day to do 6! lol, was working 15 hours a day as it was!