I've just tried a simple test in WB: ten pairs of Mech Inf (with Combat 1, due to a poor choice of leader), each pair attacked by (1) a single Cat, Treb, Cannon, Art'y, Mob.Art'y, all unpromoted, and (2) same siege weapons but each with Barrage 3 which cancels the Combat 1. In that order, the strengths left for the collaterally-damaged Mech Inf were 30.7, 30.7, 30.1, 29.8, 29.1; 29.1, 29.1, 28.2, 28.8, and 28.5.
Every one of the ten attacking siege weapons died in the assault.
Only the Mobile Art'y. scored hits on the units targeted (1 hit and 2, respectively, for 16HP damage each)
I also tried attacks with a Tank and a Mod.Arm., both with Barrage 3. The tank reduced its collateral adversary to 29.4 strength, the Mod.Arm. to 28.5, and both died in the assault, the latter to a series of 6 hits at 17 HP each against 80.2% odds: the theoretical chance of that happening is 6 in 100,000.