I was in the same school of thought as you guys but then i realised during a match how everything in the doctrine just clicks.
If you use the flares. not only is it better than a recon run but it lasts quite a while and it allows your sexton to dish out some accurate fire thanks to the vision.
The valentine is basically an 80 fuel sight track that can defend itself and the sexton. Think about it. it has smoke to defend your sexton if you get rushed. the wallhack lets you see where his troops are worth bombing and the vet 1 ability lets you override the cooldown on sexton.
The 100 muni arty is amazing for the simple reason that you can call it down through the fog of war.
The only thing that's meh is the 12 cp sector arty to be honest.
This doctrine works best vs okw and going for comets with piats on sappers. piats are awesome btw.
Tl;dr sexton is pretty good with the doctrine's perks. OP munis and have fun.
I agree with you on performance, but by the time I'm even able to build a valentine I could have a cromwell instead. It's really hard to justify investing in a valentine + sexton combo at that stage of the game.