Hi,

Replacing the pedals with MTB platforms might be an idea.

After that I'd suggest your cranks are fine in that the flexing
is probably more the frame than the cranks, and updating
them will not help that much, but I'm not sure ....

Though at even odds I'd put money on stiffer
cranks not helping at all any day of the week.

So IMO very probably a waste of money.

rgds, sreten.

I have something very like these on my folder :
http://www.ebay.com/itm/New-Bike-Bic...item2ec5a18a66

But a full length spindle may be better for the heavier :
http://www.ebay.com/itm/WELLGO-PLATF...9d4c01b8f#shId