Hi, welcome to the forum.
I think Stormies, Mules, or Riff Raffs will all do a very good job, but slightly different flavours. I have all three and one 335 - my Mules are in the 335, replacing 57 Classics. The 57s were fine, but the Mules gave me that and a bit more clarity. I owned Mules and Riff Raffs at the time and debated long and hard over which set to put in (I didn't fancy doing the surgery twice!). I came so close to putting the Riff Raffs in because I thought I might want a harder, tighter, edge for the rocking and rolling. As it turned out, I loved the Mules in there, and I think the Riff Raffs would have been too "hard" with my particular 335.
How bright is this Rich Robinson 335? If it's quite bright sounding, then I suspect Riff Raffs might be a bit too hard sounding for you as well (I'm trying to think what I mean by "hard" - it's kinda punchy, but less able to do mellow, not much warmth, that's what I mean).
I've recently acquired Stormy Mondays and put them in a Les Paul (they replaced 57 Classics as well, and seemed a lot more of a "like for like" than 57 to Mules were, but again more clarity and expression). Stormies turn out to be my FAVOURITE humbucking pickups - it was Mules for several years. If the "Rich Robinson" sound grabs you, my feeling is that Stormy Mondays are what you want. They have a bottom and a sparkly, sweet, top, and this clarity that quite stunned me. The bottom end would get a bit flabby under too much gain, but it would do most late 70s early 80s rock - maybe not quite crisp enough for Killing Machine/British Steel era Judas Priest riffing, but you'd get away with it - it handles classic Thin Lizzy (Jailbreak, Bad Rep, Live and Dangerous), early AC/DC (up to Highway to Hell), etc.
If you like Rich Robinson's sound, I'd choose Stormies. If you like his sound but want a bit more edge to it, then I'd choose Mules. I'd probably avoid Riff Raffs for this myself, but that might just be how I play. If I plug the Stormie guitar in, it gives me "Black Crowes" without thinking, if I plug the Mule guitar in I have to fiddle with the amp to do the same thing. If I plug the Riff Raff guitar in (an SG), I'm usually playing some other style! (I've just been recording a song where all the guide guitars were the SG - I was struggling with the sounds while arranging, I've just replaced the guitars with Stormies and it sounds richer and wider, but with losing any definition).
My gut feeling, though, is that you're going to end up with Mules because they're a safer bet. If I was choosing for my 335 again, I'd go Stormies, no question - but then I know what they do and how I think they'll react with that maple construction and what I want out of it. But I'd be happy to play a 335 with either in and expect to get pretty much the sound I want (and the neck pickup usable).
Potted or not? I understand it doesn't make a lot of difference. All mine are unpotted, but then I don't use a lot of gain (sounds pretty much like the tones you use), and I don't play live anymore either.
