Question: I am amused by the story of Beethoven dueling another composer in Vienna, in which he takes the other composer's music, puts it upside down, and proceeds to humiliate him. I feel as though keyboardists with that kind of talent must exist nowadays, but they are perhaps not as popular as they once were.

Do we have such "Rockstar" pianists in the modern age? Who are they?

Answer: 
One of my favorite up-and-coming pianist/composers is Aysedeniz Gokcin. Not yet 30 years old, she's someone who combines a marvelous piano technique with impressive compositional skills. 

Here's her "Pink Floyd Lisztified," an arrangement of some of Pink Floyd's songs in the manner of Liszt:
You should definitely check out keyboardists from progressive rock bands. Prog rock musicians really introduced synthesizers to the popular music/rock scene and elicited really unconventional sounds from those synthesizers. Listen to what they'd done with synthesizers in the 70s, and you'd be amazed. That was 50 years ago!

The most famous keyboard giants of prog rock are probably Keith Emerson of the band Emerson, Lake and Palmer, known for his flamboyant live performances and unconventional techniques (e.g. sticking a knife into a keyboard), and Rick Wakeman of the band Yes, who was much less flamboyant but nevertheless a great keyboardist who created many interesting sounds.

My favorite, however, is Rick Wright of the venerable Pink Floyd; he wasn't into doing crazy, virtuosic things with his keyboards, but he was really good at creating sounds and conveying moods and atmospheres with his synths.