G'day from down under!

First time poster here and a regular reader, but now I have a problem I can't find the answer to...

I need some help with a strange (to me anyway) problem I have with my Gaui Hurricane 550.
I initially had her running with the stock 550W motor, stock ESC and 2 EVO 3S 2500 paks, that gave me a 5-6 minute flight time.

Recently I upgraged to the 1500W motor, stayed with the stock ESC for the time being, and decided to purchase 2 TP 3S 3850 paks. I did the usual half discharge five times with the new lipos before increasing the flight time, but I found that the stock ESC would cut power after only 4 minutes!

After experimenting on the weekend I have found that...with the 2 x EVO 3S 2500, I get a 5 min flight, cells down to 3.7V each. With 2 x Kong 3S 2200, I get 4.5 mins, cells down to 3.7 each.(makes sense)......But with the new TP 3S 3850's, I only get a 4 min flight before the ESC drops power!!! AND, the cells are reading 3.95 Volts?!#[email protected]?

Can anyone shed some light on what is going on here?
Thx in advance!

JP