I'm mostly concerned with my skill growth. Will forcing myself to fight a
marginally too stiff a board be likely to iron out bad habits and improve skill?

I'm in about the same situation as you, and have been wondering the
same thing. I'm down to 145lbs or so, and ride a donek wide 161. I
have no idea what weight range the wide 161 is designed for, but I'm
sure I'm way low of median.

Last season was my first consistantly riding, and I really learned a
lot. Sometimes I definately felt that things would be easier with a
softer board or a touch smaller sidecut. Especially when I try to make
carved turns in less than perfect snow, I feel like I just don't have
enough mass to bend the board properly. But at other times, I'm very
glad I have a stiff stable board... you can just set that edge in and
then lean back on the tail to power through choppy crud.

In the end, I think it's not as easy, and probibly not ideal, but I've
still learned and done well, so I don't think it's a huge barrier. I
wouldn't worry about it so much. One sure way to find out would be to
demo a board for a day. I take runs on friend's gear every so often,
and on the softer, shorter boards, things definately feel much more
mellow. But at the same time, I don't feel nearly as confident getting
a little speed. So, I've stuck with what I've got, and am still
learning, and don't really worry about it anymore .