Motorcycle Exhaust and Engine Efficiency

"engine efficency. YES or back pressure If you reduce the area of the exhaust outlet, yes.or thrust of my bike????/YES.

Cool air does not flow as readily as warmer air, so, if you remove heat from the exhaust stream engine efficiency will be reduced.

Racing engines' exhaust systems are wrapped with insulation to keep the exhaust stream as hot as possible.

Practically speaking, unless you reduce the exhaust stream flow area you probably won't notice any difference. If you reduce it, you could see a significant power reduction.