Good tip.

I've tried the same sort of thing with an IDL render, plus non-IDL... and / or plus shadows only... and blending it all in Photoshop... and it worked well.

Recently just been using inverse linear point lights, in IDL-only renders though.

Most of the time I just render in one take... and quite often don't even end up tweaking levels, tone, etc, in PS, if I get the lighting right in that take.

I've found the fast progressive preview mode rendering of IDL in PP2014 a big help with the final-render-in-one approach... and firefly is definitely speedier now too, when doing this, I've found.

But the option of mixing down in Photoshop is a great one to have... and certainly affords a lot more control over the final result too.