I see that this topic in under research, and I would like to post all the data I have found on this subject. I only know of two Battle Armor designs that use the BA LB-X AC. The first is the Thunderbird II (XTRO Republic II, p.16) and the second is the Black Wolf (TRO 3145, p.23). The Battle Value is listed for both units, and I have done my best to reverse engineer the BV of the BA LB-X AC. Roughly calculated, the number I have come up with is 20-21 BV.
I think it is important to compare the BA LB-X AC with it's closest competition. They are the AP Gauss Rifle (TM, p.318) and the Medium Laser (TM, p.317). The AP Gauss Rifle has a BV of 21 and the Medium Laser has a BV of 46.
The range of the AP Gauss and Medium Laser are exactly the same and both are one hex further than the BA LB-X AC. The BA LB-X has a -1 to hit, so I consider all of these to roughly have the same value. An undamaged Clan Point using Medium Lasers (rolling a 7 on the Cluster Table), would do 15 damage with 3 chances for a Critical Hit. The AP Gauss Rifle would do 9 Damage with 3 chances for a Critical Hit. Finally, the BA LB-X AC would do 12 damage with 12 chances for a Critical Hit. The AP Gauss Rifle has the Anti-Infantry ability, but the BA LB-X has the Flak ability.
Barring Ferro-Lamellor armor, the BA LB-X AC seems on par or better than the Medium Laser. A Battle Value of around 20 seems too cheap, and I would think it would be 35 or greater.