If you doubt the accuracy of your current instrumentation, what makes you think a $25 bicycle gizmo will be any more accurate -- or, even work properly for any length of time? Isn't this the second one you've had to buy?
Other than that, I think that any gains would be minimal for two reasons:
1) The cluster doesn't weigh enough to strongly affect performance, so ditching it probably wouldn't give you enough gains to justify the procedure.
2) Your faring covers the cluster, so if you're ditching it for aesthetic reasons you probably wouldn't notice the difference much (from an observer's viewpoint) unless you got rid of the faring as well, which would be a waste of a good custom paint job.
A free one:
Less is not always more when it comes to being informed about the operations of your bike.