I can't think of any lemon-only scented soaps that match your criteria and are currently being produced, but if you're looking for a unicorn what comes to mind is the discontinued AoS Lemon tallow formulation. Can't say I have any experience with it but there are a lot of rave reviews.
Mike's Natural lemongrass and eucalyptus is pretty darn lemony to my nose. And it's definitely top quality and high-end.
Or you can belly up to the bar and join the Arkoholics...
The art of shaving lemon--regardless of old or new formula--is, to my knowledge, the only 'straight' lemon soap around. It's excellent, along with companion lemon frags and cream.
Get arko, then get some extremely good performing unscented of your choice. Cut the arko with the unscented until you can tolerate the scent. Bam... lemon scented soap with good performance.
Just got a jar of Catie's Bubbles Tonsorial Parlour. It's performance is as great as any of Caties' and the scent is definitely lemony with a slight woodsy/spicy undertone. But mostly lemons. I totally recommend it.