I cannot find the results I got when I tried it a long time ago anymore unfortunately, but I recall seeing only a miniscule change with the unit experience, in the general vicinity of 8-14 kills without any upgrades, and 9-14 kills with 9 chevrons. The weapon-upgraded ones killed in the order of 20-35 each time, which is a very visible increase. As I only did the test 3 times with each instance, I considered the increase of chevron upgrades statistically insignificant, but perhaps the purported accuracy boost mentioned here by Blxz is true then. I suppose accuracy boost would make more of a difference in your test when each missile that actually hits has a greater chance of killing its target than in my test.

Still, the added penetration of the weapon upgrade obviously makes more of a difference than the accuracy. At any rate, the stat increase shown in the unit card for chevrons is clearly not correct.