flycbr

The essential information missing is the cell count you started with before the "switch to a 6S battery", the cell count you used with the 5055-280, and wattmeter readings for all the setups. If you don't know the expected power and the actual voltage, current and power, you don't know whether you have a fundamental setup problem, or an adequate setup that just isn't working properly, eg a weak or un-charged battery or a mis-calibrated ESC.

But even with limited information, I think you have a fundamental setup problem. At Kv of 420 rpm/V, 13x5 is too small a prop for anything less than 8S.

On the same cell count and prop, the DD motor with Kv of 420 rpm/V will use only 60% of the power the 500 rpm/V Turnigy does - so it's no surprise the performance is a lot less. The 280 rpm/V motor would just be an even bigger step backwards unless you were using at least 1.5 times the cell count, ie 6S vs 4S or 9S vs 6S.

Assuming 6S, the 420 rpm/V setup is never going to adequately fly an 8.5lb model with a 13x5 prop. Max power will be of the order of 500W which is OK for a trainer, but pitch speed will be too low at 40 mph. Less than 6S with a 13x5 will never work, period.
Assuming 6S, 13x8 would get you around 80W/lb and sufficient pitch speed at 65mph, or for more reserve power if there is enough clearance 14x8.5 would work well.

If you wanted to use 4S, you'd need to be able to use a 16x12.