Engine seems to lose power under electrical load
 

Hi
I have a 2008 rx8 231 that runs and drives brilliantly.
The only thing that bugs me is the engine seems sensitive to high electrical loads. For example I will accelerate hard and the engine will pull well. If I switch say the rear window / mirror heaters on there is a noticeable drop in power.

Does anyone notice this?
I have fitted a new heavy duty battery as the old one was pretty old, but with no improvement.
I am not particularly bothered but just wondered if anyone has come across this?

Thanks

Antony