+1 for Paul's point of finding "your" limit. Like others have expressed here, my personal limit is 200 lb. The pitch sensitivity doesn't bother me - I fly gliders with lighter to non-existent stick forces. Negative stick force gradient is indeed disconcerting. But I can tell you that an RV-8 that is almost as docile as a Citabria when flying solo can be very laterally twitchy to manage with a heavy person in the back. Altitude plays into this too, because the ground speeds are higher at touchdown, and it just makes the lateral dynamics on roll-out surprisingly fast.
On a calm day and a wide runway at sea level, sure, I can, and have, landed with someone pushing 225 in the back. But I know that if I do that regularly, at some point it is just going to get too exciting. So, 200 is my limit.