I have Irish Tours, Apaches, and Sultans. I'm tempted by Mother's Milks for the same reasons as you - more sweet top end that I can dial back and add as necessary. They all do it to some extent, but I find myself thinking "I should have gone for MMs" sometimes.
Out of the three sets I have (and they've all been around the same strats to see how they compare), the Apaches are closest to doing what you describe. They also have this low-mids punch (in comparison to the ITs upper-mid bite), I do find the Apaches the nicest for clean tones... So, yeah, I agree: the ITs are less interesting in comparison on cleans. They're not bad, just less "interesting" clean than the Apaches (and I suspect the MMs). I get the same on humbuckers - I love Mules, Riff Raffs, and Stormy Mondays, but one of them out-strips the others by far for "interesting cleans" (Stormies).
I think your judgement is probably right - the 63s I'm guessing are a bit like ITs, and I do miss some top end clarity with ITs. The MMs is what I think you probably want, but I would suggest checking out Apaches if you haven't already - they surprised the hell out of me when I finally bought them. I was expecting "Hank Marvin" and not much else... I was so wrong, they have a sweet top end, and this lower mid growl. They just work on/at everything I chuck at them. From everything I've read, though, the Mother's Milks have the most top-end clarity,
Btw - I don't think you want Sultans, they're lovely, but they're smooth and polite sounding. (Btw, I find Apaches do the best Mark Knopfler impersonations for me)
Oh - you may already know this, anyway - but whatever set you go for, I'd suggest avoiding RWRP on the middle pickup - you seem to lose a bit of that precious top-end clarity.