The Whitworth is an amazing mech. In its main configuration (LRMs), it sits well in the company of Archers, Crusaders, Warhammers, Marauders, Awesomes, Stalkers, you name it. My best use of it in recent memory is as an Archer-substitute for medium brawling lances with Enforcers, Centurions, and the occasional Vindicator. Teams like this, particularly when they all jump and have the terrain in their favor, can put a serious hurting on even heavy enemy lances through shrewd combinations of indirect fires and rotating units into cover to present new armor while focusing fire.
That said, with the single-heatsink problem, the most promising of the upgraded versions is lackluster at best. The Capellan version, while cute, is a one-trick pony; best used, as has been pointed out, to deprive a combined-arms player of his supporting arms. In the same role, I would tend to use a whole bunch of BadgerDs and carry along some BA for good measure.