Regarding the exaust back pressure.

I have been wanting to do some exhaust back pressure research and I do not know of a suitable gauge to test this, maybe you could recommend the gauge you use and where I could purchase one.
I believe the back pressure should be minor and I dont know how exhaust gas temperature will affect normal gauges.

The info I have accquired so far regarding exhaust back pressure results comes from a reliable source. I got this info from a course I was on recently. But I have not proven these results for myself yet.

On idle 0-30mBar is good
50mBar is border line

Under load
up to 100mBar is acceptable

If I have converted this correctly 100mBar =1.45psi

So I would say that you do have a seriously restricted exhaust system.

hth
James