Act444, this is a totally different proposal which only you can say if it makes sense or not, but I will mention it in case you didn't thought about it.
Currently you are using the 60D and tamron 17-50 2.8 @ 5.6 for this particular situation, and would like at least the same wide framing, and better reach you currently have, and you already have a good collection of FF lenses including the 24-105.
So, if you sell the Tamron, 60D, and add the budget of the new lens you intended to get to improve your current situation, and get a 5DII instead, The 24-105 will act as a 15-66 on your 60D. You get better ISO and can decrease the strength of the flash, get better resolution since you will use all the glass of the FF lenses together of more pixel count. With better resolving power and lower noise, you can crop more too getting more reach, and use the lens wide open if lower noise/lower flash power/shallow DOF is ok for you.
I have read (others with a 5DII might comment on this) the AF ability of the 5DII is very good in low light. The Camera won't use the advantage of the 2.8 with the 24-105, but in no light situation like yours, the help from the flash is needed.
You will loose the articulating screen, and the slightly better FPS, but you might gain in other different situations too with the FF camera and current lens collection.
EDIT: I thought you intended to get a new lens to improve your situation, but seems I was wrong, so what I have mentioned might be an expensive alternative.