I have an 11-34 SRAM cassette on my cannondale t800, and it has the long mountain rear derailleur. I don't think that there are road derailleurs that will handle the 34t cog.
My large chainring is a 52, and the 11t cog pretty much never gets used unless I'm just slowly turning the pedals downhill to keep my legs moving. I think you could happily go with a 12 tooth as the smallest cog on the rear, but YMMV.
I have a 9 speed drivetrain, so I don't know about the availability of 8 speed cassettes.