This.
Its not that the LPL is great on paper, its where you end up "playing the game" a lot of the time.
The LRMs still soften up at range & the Elementals are LOOKING to get into SL/Flamer/MG range w/ you where the LPL will cut them apart.
actually i wasn't thinking 'better to fight clan BA", i was thinking "better to fight clan mechs in the environment where the IS actually excels, urban and very dense terrain." places where the clan advantaged of speed and range are negated, allowing the IS to fight on terms that are somewhat more even, and the IS's group coordination and willingness to fight dirty with things like melee attacks gives them an edge.
the LRM give you the option for indirect fire, while the SRM6 and LPL gives fairly high close in fire power for when a clan mech walks round a corner into LOS. the SSRM2 adds to this slightly when it works, but mostly is there to give the vehicle a longer duration backup weapon. the artemis on the LRM conflicts a little with indirect fire use, but gives the LRM that much extra power in up close direct fire.
during the invasion, the only times where the IS managed to get a relatively even kill ratio on the clans was when the IS forces withdrew into urban fighting and in battles where the terrain resembles urban fighting, like canyons and really dense forest. places where the clan's can't make good use of their longer ranges and generally higher ground speed, and where there were lots of chances for the IS forces to leverage group fire, combined arms, melee attacks, etc. in such environments the Manticores PPC would have been more hindrance than help, so the post invasion refit dropping it for a LPL, which has similar firepower but with more effective close range performance, makes some sense.
as does the dropping of the ML to save space for Artemis IV on the main missile racks. in the event of am ore open field battle, the old succession wars tactics of staying at range and using the longer range of your LRM and PPC to pick apart targets no longer works. on both the clan and IS sides your opponents are now generally more mobile and can inflict fairly extensive damage at longer ranges than you can. so you'd be using that LRM to soften the targets up as you race forwards to engage at close range with pulse laser and SRM's, trying to get in as much damage as possible before your tank gets crippled.