Hello mparker,
Since you posted this thread in the incandescent section, I'm not sure if you specifically want an incan or LED-upgrade...
For some years, I used an SL-20x with an original Streamlight SL-35 lamp assembly and a Streamlight 'flasher/dimmer'-tailcap. This is a reliable incan combination that a lot of CPF'ers have used over the years.
Since 2008, I have a Malkoff Triple Warm in my SL-20x (rated 510 lumens). This is an LED-'drop-in'. Just like other Malkoff gear, it's great and still works flawlessly. This is not a drop-in that Gene Malkoff has on his website. You could contact him (malkoffdevices.com) and ask if he can make one for you. I'm sure a Triple with the current XP-G's (or XM-L's) would even be a lot brighter. Gene Malkoff is a busy man, so I don't know if it will happen. It costs nothing to ask, though. Expect having to pay at least $150,00.
I know Terralux has a drop-in for the SL-20, the TLE-110S. Brightguy, among others, sells these for around $50,00. I never handled one of the Terralux drop-ins, so I can't really comment on them.
Succes with the search...
Regards,
Nachtwacht.