That is just it, this is just one of many places where the TT rules were pretty well balanced, based on critical hits. For IS mechs, double heatsinks took up 3 critical slots, weighed 1 ton, and had the heat efficiency of 2 standard heatsinks. So triple the space double the efficiency, at the same weight, but that triple space meant it was much more likely to be destroyed by damage in battle, causing the loss of 2 heatsinks worth of cooling, while a standard heatsink would only lose 1 heatsink worth of cooling in that same case, and is much less likely to have been the component damaged in the first place. Clan double heatsinks really removed that penalty especially in the case where you were already going to put 2 heatsinks in that component. While it is still more likely to lose a double heatsink due to damage than a single, it more than made up for that offset due to weight factor.
Double heatsinks did not obsolete single heatsinks in many IS mech designs simply due to the space issues. For instance, legs were very common location for heatsinks in IS designs, but since there were only 2 critical spots, thus when IS double heatsinks are used, legs can no longer fit the heatsinks, removing more critical space which previously would have been used for weapon mounts.
That is the whole point. It was previously balanced. But they decided to unbalance the system by simply mucking with the "double" heatsinks values and not actually giving double the dissipation, but kept all the negatives of critical slot usage and damage probabilities.
The same was done with the doubling of armor. They didn't double the ammo, when now it took twice the amount of hits to destroy a mech, causing the mechs that used weapons that required ammo to need to use more critical spaces and tons in ammo when those critical spaces and tons should have gone to other weapon or support equipment (armor, engines, heatsinks, AMS). Essentially forcing a move more towards energy weapons. This might have been intentional because no one used energy weapons because they had kept the heat values of the TT game but cut their damage output and increases their cycle times so much that if you used even used a single medium laser you had to had 8 heatsinks (on top of the ones built into the engine) just to fire it constantly without risk of shutdown...
That is the whole point. They never looked at how the TT mechanics were balanced, and would simply muck with one thing, but not look at how it affected all the other pieces that balanced that mechanic in the first place. Ok, you want matches to last longer, make sure you appropriately modify the number ammo held in 1 ton such that the same relative damage is done by that ton of ammo (case in point, if you double the armor, all ammo totals are immediately doubled, now if you modify the weapon's damage down by say 20%, the ammo total would also increase by proportionally. Take SRM6 it does 2damage per missile and there are 100 missiles per ton by TT rules, this equates to 200 damage per ton of ammo. If you reduce the damage from by each missile from 2 per missile to 1.6 per missile, the equation for the number of missiles per ton should thus be 2*100=1.6*<new number>, which means the new total missiles per ton should be 125, add the doubling of the armor and it would be 250 per ton). Then you have to look at the affects on other things, like anti-missile systems. They too have a set amount of ammo. With so much increase in total number of missiles needed to be fired to produce the same damage, the damage reduction per ton of AMS ammo has been decreased, which means you need to increase the amount of ammo in 1 ton of AMS ammo to compensate. This gets more complicated due to the fact that AMS works on both short range and long range missiles. Even more complicating the fact is that AMS should only work against 1 missile system in a round (i.e. you get fired on from 2 mechs, one used a SRM6 the other an LRM20, your AMS will only work against either the SRM6 OR the LRM20, not both). This makes logical sense, in that you can only fire at one group of missile at once since the single gun can not aim at multiple locations at the same time. I don't even think they have that rule in place in the video game, it simply works against all of them until it runs out of ammo. Lets not even talk about laser AMS systems and their heat....
This is the whole point. TT rules were balanced. You can modify them to make things better from a user standpoint, i.e. 10 second rounds do not translate well when someone hits the trigger and then has to wait 10 seconds to fire again on every weapon in the game. But there are simple solutions. If you want to increase the cycle time, do it properly. For laser weapons, if you want small lasers to have a 2 second cycle time, then divide the damage and heat produced by 5. For ammo using weapons it is a little more difficult, as you need to also perform the ammo per ton calculations I previously posted when you cut the damage of an individual missile.