Engine lacks power + poor fuel econ

check map sensor and temp control sensor

What do you mean by "full tune up?" Plugs, wires, ALL filters? If so, yeah, then check the MAP and TCS.

Also check TPS as well as the 02 sensor, just 'cause it looks new, doesn't mean it's working properly.

I would first do a compression and leak down test, to see if the motor is internally healthy. If those tests go well, do a basic tune up. NEW spark plugs, new plug wire, new cap, new rotor. Dont cheap out here. Buy good parts, or you will end up doing this again in a couple years. Buy NGK plugs and wires, bosch cap and rotor. These are brands that ive had very good luck with, and what i run on my 575whp B16, so I think they will work for your stock z6. If you are un sure of your timing, what have you done to change it? Have you removed your distributor, or replaced the timing belt? Get yourself a timing light, and adjust the dizzy to correct setting. If you are unsure on the timing belt, thats a bit more trouble. Remove the valve cover, and the upper timing belt cover. Then line up the TDC line on the crank pully with the mark on the lower timing belt cover, then check to see if the cam gear is also showing TDC. (TDC = top dead center)

I concur with Joel; his advise is the best. I'd only add, is the check engine light on? If it is, retrieve the code and address that issue. Your Cat Converter could be going out.