I guess there could be two meanings to your question.
One would be the maximum muzzle velocity attainable with a tuned gun.
The second - and related - would be concerned with the ambient temperature at which the gun is being operated. As all CO2 guns experience an increase in muzzle velocity due to temperature, you will gain about 2fps for every 1 degree F increase in temperature with the QB79. So, shooting at 85 degrees F will give a muzzle velocity of about 40fps greater than shooting the same gun at 65 degrees F. At least that's the case until the temperature reaches about 95 degrees F - at which point "valve lock" happens with a factory gun - and the muzzle velocity falls rapidly with increasing temperatures.
The second point further relates to the first in that any given muzzle velocity attained by a tuned gun needs also to have the associated temperature at which the muzzle velocity was attained. For example, we're seeing about 640fps at 65 degrees F from the QB78 and QB79 in .22 caliber when fitted with an XP Tune Kit and XP Tuned Valve. That would be nudging 700fps on a hot day at 85 degrees F plus. And that is attained when shooting 13.8 grain wadcutter pellets.
Wait a minute! We also need to specify the type of pellets used too as lighter, pointed ones will tend to shoot faster also.
I know there others on this Forum who have good data and I'm sure they'll add to this string...
Archer Airguns Inc.
Online stores http://www.archerairguns.com