I seem to recall a 1mph difference on that load. I'll check my spreadsheet when I get home.
For an actuary the value of 1 mph difference while traveling 340mph may be relative if the distance traveled is hundreds and hundreds of miles but in AH it is irrelevant. The acceleration, climb, and turn are an issue though. As I dug through my notes on the differences in speeds and climb rates for the different ordnance packages on the Mossi Mk VI, I did come across a little note of mine saying "less than 1 mph" difference with only the 2/500 lb bomber vs having an empty bomb bay. Mind you, that was testing with 25% fuel so it might be larger if the Mossi is further weighed down by larger fuel amounts. I didn't test that.
About the only planes in AH that I take in to consideration of "just how active will I be in fighter mode after the ord is gone", are the Mossi and P38. If while in the Mossi I'm going to be providing top cover I'll take the 2 bombs and leave the rockets (and racks) behind, the P38L not so much but I will be much more careful about tight maneuvers. The rocket racks on both add a major amount of drag that not only do the flat numbers show the difference (speed and climb) but the handling is noticeably different as well.