I think a real custom builder should be able to built the right bike for you including taking stiffness into account. A friend will be ordering a RIH soon. He discussed he will get a roadframe with the triple triangle alike design that RIH normally uses for their trackbikes. It will be the 1st time RIH will make a road-frame with that characteristic. He explicitely wished it. Other special requests they will fulfil is full chrome, unpainted lugs, translucent paint and name engraved in bracket shell. Hence proper custom.
Without any doubt the frames by Duell, the bike I posted in my previous post, will offer a great responsiveness/weight ratio as they are designed as serious bikes for modern competition.
On John's comment on that Max tubing is still available: I recall a fellow member on here told us Zullo of Italy had still Max tubing available and would be willing to build frames with it.
I have a Batavus Professional built with SP. I haven't built it, nor ridden it, but I have been told SP can be considered a stiff type of tubing from yesteryear.