Backpressure is the pressure from the gases that are left in the
combustion chamber at the end of the exhaust stroke. Many engines
are designed to take advantage of this backpressure, so that the
residual gas and the gas taken in during the intake stroke
contribute to the pressure in the chamber at the point of
ignition.
Extractors help exhaust gases flow out of the combustion chamber
more freely, so that less gas remains in the chamber at the end of
the exhaust stroke. In turn, there is less gas (and less pressure)
at the point of ignition, which results in less force pushing the
piston down.
Extractors give you an advantage at higher revs, where it allows
the exhaust gases to be pushed out faster by a faster-travelling
piston. However, because of the lower compression at ignition,
you'll experience less power per stroke, and without the advantage
provided at higher revs, you'll find that the engine will be more
sluggish at lower revs.
Basically, you're left with only two corrective options at that
point: either go back to the old exhaust, or modify your intake so
that more gas (in the form of fuel-air mixture) is taken in with
every stroke, thereby increasing the compression at the point of
ignition.
I haven't had any experience doing this with a fuel-injected
engine, but with a carburetted engine, modifying the intake usually
means getting a bigger carburettor. You can turbocharge or
supercharge the engine, but that becomes a question of whether it's
worth the cost and trouble.