I am very sorry to hear that you're having trouble with your M5.
I have not seen evidence that any Eisenmann systems for the F10 M5 lose power or torque over the OE exhaust, both in our own testing at AMS, in testing at Eisenmann's facility in Germany, and in independent testing by Eisenmann customers worldwide.
Our own test results can be seen here:
Were you able to test the M5 before and after the exhaust install to check power output? Any noticeable reduction in performance should have shown itself on a dynamometer, and with a dyno graph in hand we could help you in diagnosing your issue.
One area I would suggest checking is the vacuum line running to the rear mufflers. If the vacuum lines are not plugged during the exhaust installation your M5's performance can be affected significantly. It is possible that these were left hanging and open during the Eisenmann install, and then re-attached to the OE BMW valves when the OE exhaust system was re-installed.
Installers frequently omit installing the vacuum line- so often, in fact, that we have actually moved to attaching warning labels on each exhaust system we ship:
"All Eisenmann systems not using the OE vacuum control system must plug and close the vacuum lines at the rear of the car. Failure to do so will result in reduced power and ECU trouble codes."
Ultimately it is very difficult to say exactly what happened without hard figures or testing. A simple lap around the block does not give us enough data to diagnose the issue.
With regard to tip size and it's effect on power output-
Eisenmann tested each tip size in-house for not only power output but decibel output (as required by the German TUV), and found no difference in power output with the larger 102mm tips. A transition from a smaller pipe in the muffler to a larger pipe at the outlet will not reduce power output, as it will not restrict exhaust flow.
As a thought experiment, picture which exhaust system would create more exhaust restriction- one with 20mm exhaust tips, or one with no exhaust tips at all? The 20mm exhaust tips would create a major restriction in flow, while the system with no tips (or infinitely large ones) would not have an impact on power output.
Exhaust tips are more of a cosmetic finishing item than a contributor to the power output of an exhaust system.
Once again, I am very sorry to hear that you've had trouble at all, whether the trouble was related to the installation of the exhaust or not. We are here to support you in any case, whether you'd like to re-install the Eisenmann system and pursue a solution to the problem you felt, or whether you'd like to simply send the exhaust back for a refund.