I did a cursory lit review using the official published performance data from the developers and compared it to the data available on Mike Williams' excellent site for the Bf-110G-2 which is what I imagine is the model giving most people trouble online.
Maximum speed of the Bf-110G-2 is 489 km/h in Combat mode (1.3 ATA, 1310 horsepower)
Best sustained climb is 15.6 m/s
Maximum TAS occurs at 6500 m in what I assume is the standard atmosphere, where it will reach 581 km/h TAS (again at 1.3 ATA)
From Mike Williams' archives:
First document: http://www.wwiiaircraftperformance.org/me110/Bf110G-2_data.jpg
Maximum speed (it doesn't say but I'll assume TAS) is 571 km/h at 6500m (a 1.7% difference from the game)
On the deck that aircraft flew 462 km/h (no word on atmospheric data that I can see but I don't speak German) which represents a 5.5 % difference from the game
I see some time-to-altitude data but no actual climb rates
Second document: http://www.wwiiaircraftperformance.org/me110/me110G-2-chart.jpg
Maximum speed appears to be at 5800m where it flew 505 km/h, and this particular aircraft only flew 430 km/h on the deck which is a solid 7% slower than the previous data. Anyone have any details on this test? Atmospheric data, aircraft condition? If I'm reading the chart correctly this was at kampfleistung which should be 1.3 ATA, so I'm curious why this test underperformed the previous one. Anyways, taking this at face value it would mean our in-game 110 is some 13% faster than this test data at altitude and 12% faster on the deck.
I see about 12 m/s climb from the deck but again this test underperformed both other historical tests by a fair bit so without additional context I'm not sure how reliable this number is.
About the only things I can see to explain the difference (again, my German sucks) is that this aircraft appears to have been flown some 950 pounds heavier than the Me-110 in the previous test, which might be because of what's listed on the second page: It's smudged but it sure looks like it was equipped with the werfergranate which would add extra drag and would explain the low speed and poorer climb.
Third document: http://www.wwiiaircraftperformance.org/me110/Me110G-level-speed.jpg
This is a level speed-only chart but it does show that an 8200 kg Me-110G was flown at about 495 km/h on the deck and 565 or so at 6000m (again nothing about atmospheric data that I can see, maybe this has been corrected to ISA?). On the deck this aircraft is faster than our in-game aircraft by about 1.2% and slower at altitude by 2.5% or so.
Fourth document: http://www.wwiiaircraftperformance.org/me110/Me-110-G3_Flugzeug-Daten.jpg
I'll just take the speed data here and note that this 110G-3 (which was the long range recce version) did 561 km/h at 5800m.
Some takeaways: The differing altitudes at which the tested (real) aircraft achieved their maximum speed could be attributable to differences in air temperature, as a few dozen degrees can easily skew altimeter readings by a thousand feet or more. So forgetting the altitude, a first-pass guess without delving into DB605 supercharger specifics shows that our in-game Bf 110G is between 1% too slow and 12% too fast on the deck, and between 1% and 13% too fast at altitude.
That second test seems like an outlier, because the other 3 test data documents I've linked to here all seem to agree pretty readily with each other, and it's hard to tell from the smudge on the page but it looks like that aircraft had under-wing nebelwerfers so at least as far as level speeds our Bf 110G in game appears to be within 5% of historical performance.
The only climb rate quoted in meters per second was from that second test which seems suspect, but perhaps someone can go do some sawtooth climbs with werfergranaten equipped and see if they get around 12 m/s which might solve that mystery.
Bottom line: The in-game Bf 110G-2 agrees within 5% of historical speed data, and further investigation is required into the climb data but if I'm right about the nebelwerfer then I'd be willing to bet our 110 is right on the money.