and was wondering if there's any significant benefit in terms of heat or power consumption with 1.65v vs 1.5v vs 1.35v?
Last year THG stated that a set of low power Kingston DDR3s (either 1,25 or 1,35V) would boost system efficiency by a small margin (probably around 2% on average), with reference to 1,5V RAM, with a lower power consumption of about 0,5-1,5W at idle, up to 4-6W under heaviest load (average was 1-2W IIRC).
Several sources also report that Kingston LoVo DDR3s overclock better than 1,5/1,65V RAM, but then you'll lost the power savings advantage.
So, is there any benefit? Definitely yes. Is it noticeable? YMMV: it will be just with very low power systems IMO, so it will be not dealing with an high end Phenom II (maybe furtherly mated with an high end GPU).
Eventually, if your primary concern is savings on total expenses, you would do better going with standard voltage DDR3, IMVHO.
That said if you trust any how what THG said (personally I don't trust THG so much, but it does lots of articles and it's often one of the very few sources available about several arguments).