Andrew R Stewart 

With out actually testing a set up it's hard to say for sure how well the new ft der might shift. But here's a few comments any way.

The ft der likely has a inner plate height designed for the 10T difference. For the outer plate to clear the top of the big ring and still have the inner plate clear the middle ring. The 10t is not written in stone but only a 5t difference is a lot of variance from the spec.

The modern dirvetrains are designed around a shift gate/lift pin technology. So the amount of overshift the lever gives the der assumes that the chain will be grabbing a lift pin. Additional to this aspect of good ft shifting is that the der movement and cage shape is designed around specific ring to ring spacing. Some of the old cranks had much larger ring to ring spacing. So besides the possibility of the der not moving far enough to have the chain engage the next ring there may very well be the chance of chain rub after a shift. Remember that the cheap Shimano ft levers don't have much trim capicity.

You can modify the rings with filing down the tops of a couple teeth at a few vpoints around the ring. Shimano use to call this "W" cut back in the early 1980s. I have done this to some of my non pinned rings and have gotten better shifting response. Only about 1/4 of the tooth's height need be removed. I did 5 pairs roughly aligned with the crank's arms.

You can also change the cable pull/der movement ratio by attaching the cable on the underside of the der's anchor bolt. Shimano used to need this when friction frame mounted levers were coexisting with the early STI levers.

Two ultimate soultions are go to bar end levers which use friction front. And/or replace the crankset with a current one. Andy.