Also comes down to the range brackets, the LPL is 3/7/10 . . . which means you are getting 1 point less damage with the same TH numbers at 4-7 and 8-10 hexes as the weapon you are replacing.
Against some of the better mechs that are being mentioned? The Stealth Spector (Stealth offsets that -2TH), Havoc (4 ERML), Cougar D (8 hex short range & 1 pt more damage than the LPL), Wolfhounds (ERLL/ERPPC see range comments above), Kurita's Rokuwhatever (halves damage and spits back as much plus melee), and the crazy guy using a Hollander.
I think the Panther is outmatched in 3140s in light duels. I still think its a valid unit on the battlefield however.