X5 sluggish and low MPG

[szisis]

After an extensive service (valve cover, valley pan and intake manifold gaskets, spark plugs, coolant hoses, PS hose, 02 after cat sensor, air boot, thrust arms, radiator, etc.) my car now occasionally throws me a p0170 code and generally feels sluggish and dull. I have an average 8 MPG, without flooring it, but what gets on my nerves is that it does not accelerate as it should, especially after kick downs. I have 93k miles on it.
I must add that i've added the pre Vanos intake and a Flowmaster setup.

Any help would be much appreciated.

Regards,

Sam