Well I've been writing stuff for the kobo touch without too much hassle. It's running a fairly complete linux and it uses standard device interfaces such as /dev/fb0, enabling telnet is trivial so it's easy as to investigate and code for. The kernel source is readily available and the drivers even documented by mavell. The machine is reasonably snappy (but slow disk), and the touch input device quite fast and fairly accurate.
But unless you're writing a turn-key application or just for fun, android or some other open platform would be worth waiting for. Although you could probably get a version of android to run on the kobo touch, the e-ink would probably force some changes - all those annoying animations and interactivity responses become much more than just annoying on e-ink.
There are many reason 'apps' have become so popular, and performance is definitely one of them.
Doom on a browser? Err yeah, so it should do. Based on
http://www.7-cpu.com/ a modern cpu is about 1500-2500x faster than one of the age it was released. Even the kobo touch cpu is 25x faster than a 486-dx2-66, and iirc doom ran quite well on one of those.
It might be fun and cool, but hardly impressive when taken in perspective. It will be some time yet before a browser can run uncharted 3 ...