I can’t find a solid answer on this. I’ve tried both special selection 2014 and the regular cab from 2017. Really enjoyed both bottles but I’ve read some things that suggest that they use Mega Purple in their wines. Is this true? Is there a confirmed source on this? How much does it really matter?
It's highly unlikely that you'll get a confirmed source on this. Wineries, particularly higher end wineries are not forthcoming about their use of Mega Purple, when they do use it.
How much does it matter? To a large extent that depends on what you're looking for. If you're looking for a wine that's a pure expression of the Napa terroir and the barrels it's aged in, obviously the addition of Mega Purple (or non-Napa grapes, or oak chips, or any other additives that result in the end product the vintner is looking for) will detract from that. In the case of Mega Purple specifically, it adds a darker coloration but at the expense (to some degree, how much depends on how it's incorporated) of aroma and potentially added sweetness.
If, on the other hand, you're looking for a bottle of wine that you particularly enjoy, then it doesn't matter at all. If the color and the aroma and the flavor please your tongue, than to me, that's good wine. I personally am a lot more worried about how my wine tastes to me than I am about what went into making it.
So, if you really enjoy them, I'd suggest going ahead and continuing to enjoy them and don't worry about. If it still bothers you, find something else you enjoy more, and buy that instead. That's one of the best things about wine...There are always new amazing options just waiting to be discovered.