Wow... Larrabee and KSand24, thanks for your replies! I figured the online application process was somewhat new but didn't realize the gold priority service had only just started. I will definitely keep everyone posted as our application makes its way through the system.
We too felt it was a truly excessive amount of documentation but trusted our solicitor, as they came recommended from the University of Leeds, where my partner starts as faculty in January. We've been in a relationship for four years and living together for over three. The documentation was a challenge since our house here in the states is in my name and we never bothered updating the utility accounts, getting paper bills, etc. We included letters of support from our families, our Facebook.com/US feed, hundreds of photos captioned and dated, proof of joint travel to a dozen countries plus domestic airline tickets as well as bank statements, joint credit card bills, car insurance, W2s, paystubs, and other random official correspondence plus some online insurance printouts which they may just ignore. I suppose this strategy could backfire since it'll take a while to go through but we figured it'd be better to go overboard. It might've been easier just to get married, but we want to do it properly and didn't want to be pushed into it for immigration purposes; moreover we thought it'd be suspicious if we did it right before applying for a visa.
We did get an email from Sheffield in less than 24 hours after the appointment in DC, saying:
"Your online application has been received at the UK Decision Making Centre. Your application is currently being prepared for consideration by an Entry Clearance Officer."
Fingers crossed!
-Kelvin