SCR retrofit kit

You might have trouble integrating analog heat with what sounds like a more-or-less standard A/C setup. I have seen interface boards that will covert staged calls into fixed analog output voltages (but have never used one), that would allow you to use a standard stat. There is a model of the T775 that has relays as well as analog output, they can be massaged into providing staged cooling with analog heating, but they don't have quite the same programmability as a programmable stat & of course look nothing like a stat to most "users".

As for the economics of it, I'm not sure there is any. The SCRs merely shorten the ON-OFF cycles from the cycles-per-hour that a stat uses to what amounts to cycles-per-minute. If you need 10 kW out of the heater to meet your heatloss, it shouldn't make a difference if that's 40 minutes ON out of every hour or 40 seconds ON out of every minute over that same hour. You may see a comfort difference, however, as that 15kW banging in and out will result in the discharge air temp going up and down -- the analog output should settle down to a fairly steady discharge temp.