The DeltaT is negligible whether you connect series or parallel. I have my 3x 7970s in series and they have less than 2 degrees difference between them.

Parallel only makes sense when you have super-high-restriction... if you're running a CPU block, that will completely dominate the restriction so 2 GPUs in parallel or series wont change much (except the parallel loop has much higher chance of leak).

Here are my 7970s running full-tilt.