I'm glad to see that you're so content with your gear.
I bought my first Pentax DSLR in October 2007 and have gone through a lot of buying and selling, but now I'm finally starting to feel content with what I have--we'll see how long that lasts after the PMA announcements.
If you're feeling a small LBA itch, it wouldn't hurt to scratch it at all. The FA43 is an excellent lens, but not cheap at nearly $400 new. If you're looking to spend less, then the FA50/1.4 is an excellent lens for $200 new. For even less, you can pick up another fast autofocus 50 like the FA50/1.7 or F50/1.7 (around $150). If you don't mind manual focus, then the A50/1.7 is an excellent deal at under $100. Besides learning how to get creative using 1 focal length only, these lenses are a good bit faster than what you have, making them great choices for handheld lower light photography.
HTH,
Heather