How can I know how much badly the performance has been reduced as a result?
You can't - at least not with out a meaningful baseline to compare it to.
E.g should I check 0-60 Times are consistent with factory
0-60 times are very rarely going to be consistent with the factory times. Too many variables affect this - unless you know the full conditions the times were recorded in and and have a way to reproduce them (car, environment & driver) comparing to the factory time is going to tell you next to nothing. If you had some good pre-incident timing data of your own you could compare with maybe you could glean something I guess but this is a blind alley IMO.
should I check compression tests
Compression tests might give you an idea if you've done some particularly severe levels of wear/damage.
or is there know way to really know?
Probably the best way to directly measure the effect on performance of the engine before "X" and after "X" is before-and-after dyno runs (more realistically a rolling road then a bench dyno for obvious reasons). If you've got a before plot then try and recreate the circumstances as much as possible (same dyno, same weather conditions) and compare. If you don't have that then you can always compare to a stock plot if you can find one - but this is going to be less useful since you don't know how much your car differed from stock in the first place.
Assuming this your Civic VTi then according to Automobile Catalog the stock plot should look something like this:
A major loss vs stock indicate something was wrong with your particular car but not with sufficient certainty to pin it to the low oil running.
Car is still running.
This is really the important bit, running the oil low for a substantial time period will have done some increased wear at least, but if it's still running then it's not f##ked and all you can do is keep on top of the oil better in the future and hope it stays that way.