This does create a problem for the battle cannon. It is now the most expensive gun you can equip a leman russ with. After the demolisher got a huge point reduction, it is now the most expensive gun to be used by the leman russ. It is still useful, but it begs a question. Is it worth taking a battle cannon over a basilisk or executioner when it's 2D6 shot (grinding advance) S8, -2AP, D3 damage? It gets the same number of shots, and strength as an overcharged executioner with an inferior AP and an inferior average damage output. The range is the only real utility it has,but that's beaten by the basilisk which shoots a S9 -3AP D3 damage round and it doesn't have to expose itself. With a 5 point reduction to the executioner and 10 points for 2 plasma sponsons, the battle cannon is obsolete.
Obsolete is a strong word. Now the mathematically inferior weapons are more in line by comparison, creating a more equal playing field for your weapon options. It's no longer BC all the time every time, at least not on initial reactions.
Battle Cannons are still the tippy top on math, and now you pay an appropriate cost for it in comparison to the others.