On the subject of handhelds:
A properly designed handheld weapon-utilizing mech will always be superior to other mechs in their weight class. Clan Mechs even more so than IS, and similarly, it works up to heavy mechs for clans and mediums for IS. Assaults and IS Heavies just can't cram their weapons into the 6 crits of a torso cockpited head and two legs.
Not only that, but handhelds can punch ABOVE their weight.
For example, using a clan chassis, put 2xERPPCs in the head with use of a torso cockpit. Now sink it to capacity, that being 32 when including movement.
Let's say that after whatever armor and speed and such, you have no more weight or crits.
You still have a good 4-6 tons in your handheld weapon now. That's CLRMs, ammo, and a ton of armor for it. And remember, it can be ANY type of ammo.
Heck, you can do with just one ton 3xMGs and 16 shots of MG ammo for the clans. <- This is how I got my MBT mech to work out.
Now that's not only a good extra bit of firepower, but because of the HH using Vee rules, YOU HAVE NO EXTRA HEAT.
You can fire the dual PPCs AND the LRMs. It's a win-win.
I even made a Mad Cat successor that beats it in every way based on this principle.
On canon mech designs, you're not so fortunate in most cases. Though if you can get the right omni, usually medium or light, you can do the same thing while dealing with the only 1 crit in the head from a regular cockpit.