I've been playing an XL + 4xLRM5 King Crab for awhile now, and it absolutely rocks. For awhile, I was rocking 4xLRM5, 4xMPL, and 2xLB10X @ 360 XL - which was hilariously fun, but then I realized I was basically just explicitly vulturing everyone else's kills.
Now I run 3xMPL, 1xLL, 2xUAC5, 4xLRM5 and I regularly get 3-5 solo kills in a match. The LRMs are great for three things: softening up targets in the beginning, while I'm still plodding towards the engagement; absolutely decimating enemy scouts; and area denial of the enemy's main forces while our team converges on our deathball rendezvous. I have my LRMs set to chain-fire, so that I can pour an endless stream of missiles at anyone unfortunate enough to get caught in the open; it also means that when someone pokes their head out, I can throw them an "INCOMING MISSILE" warning while only wasting 5 rounds of ammo.
I generally play as a missile boat in the early game, pouring rounds into whoever tries to engage the main forces. I also deliberately try to set myself up somewhere where I won't be *too* hard to find, and where I have good back cover - that way, when an enemy light decides to pick off the missile boat, I can surprise them with my brawling weapons and then go back to lurming once they're dead. Once I run out of LRM ammo, I switch to dedicated brawling - by which time, the enemy is usually wrecked enough that I don't have to worry about wading into a deathball.
A single LRM5 is only 3 to 4 tons, while two LRM5s is only about 7; with some assault 'mechs, I run out of hard points well before I run out of tonnage, and SRMs don't do very well when I turn like a whale.