The finned coil idea is a good one, the problem is the lower temp brazing material used may contain cadmium as an alloying metal, not a good thing for potable water.
What flow rate are you looking to achieve, and at what outlet temperature ?. Having built and used the tubing versions, 165 degrees at .75 gpm with a BG-10 firing it is possible. Temperature is controlled mostly by gas flow to the burner with water flow held steady, and when making superheated steam, outlet temperatures over 300 degrees have been seen.
With steam injection RIMS systems, you can go from strike to boil without changing boiler connections as all the brewing water is always headed for the mash tun. Wort boiling is not a worthwhile effort for a flash boiler, direct fire is faster and more efficient than a steam heated coil in the wort which has been tested.