The thing with a singlecoil, through a moderately distrted amp is, while output does drop, it's also MUCH less compressed. Back when I had my 7620 (which was wired with a three way and a coil tap, air norton and tone zone), while there was a noticeable volume decrease in the clean channel with the pickups tapped, with my lead settings (Mesa Nomad, 3 vintage, gain about halfway), there was no percieved volume drop on leads simply because the attack was so much mlore explosive, due to the uncompressed nature of a singlecoil sound.
And, for those "in between" sounds, you really need a single coil that balances output-wise with the tapped bridge pickup. A single-coil sized humbucker's definitely an option, provided you wire it to be tapped as well in position 4, or otherwise the sound will be too unbalanced and won't give you that strat "quack."
I set my UV's single-coil height by dialing in the position 4 sound. I don't use the single on it's own very much - personally, i could care less aboput the output, I just think it sounds sterile and compressed for a single - so for me, this is what makes the most sense.
But anyway, that's why they didn't use a hotter pickup - because with even a light amount of gain the amp's compression should effectively even it out, and because a hotter pickup would throw off position 4.