If you've got EFI, then you definitely want the lambda sensor regardless of the presence of a cat converter.
The sensor tells the EFI computer what it needs to know to adjust mixture, and without it, most systems revert to a significantly rich mixture across the board in an effort to not burn the engine up.
With it you get:
Stoich mixtures during closed loop operation
More accurate mixutre at idle (even though that's an open loop function, the ECU recognizes that the sensor is present and functioning and uses an appropriate fuel map instead of defualt 'emergency map')
What you get without it:
Slightly to horrifically over-rich mixture at all reve ranges and throttle positions
No stoich during cruising (resulting in a significant drop in miles per gallon)
Loaded or nasty plugs making for worse runing yet.
Do yourself a favor and leave it in there, really.
