The biggest disadvantage to AC/2's and 5's plus PPC, is in the beginning of the game. Your pilots simply don't have the gunnery to make up for the minimum distance penalty. Since ranges in the start are pretty close from what I've seen on the streams, due to large number of lights, vehicles and medium mech's that closes very fast. Add the penalty to AC's for firing twice in a row, and they are simply not worth their tonnage, as you will likely face very low percentages to hit. Later on, you don't want AC/2's because they don't pack a punch, while AC/5's might be useful in conjunction with a PPC on a medium later on. LRM's are the exception regarding minimum distance. They don't heat as much, each missile gets a ToHit and the in-direct fire is gold.
As soon as possible at start, I will equip my Blackjack with 2xLL, 2xML, strip the jump-jets and throw in a bunch of HS. Reason being, that I won't be firing those AC/2's close up anyways, leaving me with 4ML's in stock build. My approach will have more firepower up close and long range compared to stock, while not being totally heat neutral, it just means that sometimes I will have to forego firing one or two ML's, and still have the same firepower as a stock BlackJack (which will also build up a lot of heat just from 4ML's and less HS)