Gary,

Unless your mixing chamber is very small, your flow rates very high or
you have laminar flow through the mixing chamber I doubt you need to
improve mixing.  If you do need to improve mixing I'd ensure that you
have highly turbulent flow in the mixing chamber (add tortuosity to the
path), I don't think that you'll actually increase mixing substantially
by increasing the gas temperature.

However if you really want to increase gas temp I'd purchase some heater
tape and wrap the incoming gas lines, that way you maximize the heated
surface area the gas sees.

Good luck,
Matt