You're posting twice - in BromptonTalk also. The Brompton crankset is light but not particularly stiff. The 3 challenges you will have is chain alignment, weight saving, and getting the right size teeth.
I investigated doing better, looked at the TA Vega, but its only slightly lighter and a high cost.

If you don't get chain alignment same as Brompton, any small % you save on weight you'll lose on chain drag.