yeah. timbuk2's are great if they're sized right for the load and you use the cross strap.

otherwise, you are kinda stuck with a bunchy shirt. i never do anything with excess strap loops. the newer timbuk2's(2010) have a cam buckle quick release, which is nice, but i don't know if their extra features will help your problems.

i got a kemmer bag chrome copy and it's so much better of a design that i sold my timbuk2 messenger and picked up a swig backpack. the backpack, aside from looking pretty huge on me and being difficult to access without taking off completely, is awesome- comfortable at any load and just as durable as a normal classic messenger.
but as far as messenger bags, timbuk2's are very hard to wear if they're not loaded in a particular way with the right amount of stuff for the size they are. and think of the fit like shoe laces... you can adjust them a great many ways but you have to find what's right for you for it to be comfortable and snug enough that it won't fall off.