I’ve also found exactly the same thing 2many62s describes especially on cold mornings.

“……If the car is equipped with a factory type thermostatically controlled choke, the choke will take way too long to pull off. If you have an electric choke, it will pull off too soon. Plus, blocking the cross-over didn't make any noticeable improvement after the engine was "warmed up". I think this is one of those questions to which the answer is, it depends…….”

While not as bad as eliminating the heat crossover, eliminating the heat riser will also slow the warm up period considerably. None of my cars have a heat riser valve and a few in the past have run manifolds and/or heads without the heat crossover. As I often drive my cars year around and during the winter and we get some pretty chilly early mornings I can say first hand it can be a real PIA.

The one band-aid I’ve found that really helps during the warm up period is to add an MSD box. The multiple spark during low RPM really makes a big difference in firing off the cylinders until the car warms up.