Mods to Jamis Aurora Elite Drive Train
 

I have a 2009 Aurora Elite, it has 105 STI 10 speed shifters, Ultegra RD and a 12-27 rear cog. I was thinking to put a 9 speed 11-34 cassette. If I made this change I am assuming that the RD would need to be replaced with a mountain bike RD. This would change my low end gear inches from 30 to 23.

Would the shifter work without any problems? I don't know if the cable pull is the same or would I need to get a JTek shiftmate to cleanup any shifting problems?

And if you have a better idea, throw it out here.
I had thought of using the new 10 speed cogs and RD but this seems to be a cheaper way to go.

With very few exceptions, Shiman (rear) shifters all pull the same cable, so you should be OK with that.

To make the 10 sp shifter work with 9 cogs, you may need to look at getting a shiftmate since the cassette spacing is different - OR - look at pairing SRAM Apex cassette with 9 sp Shimano MTB derailleur.

See shiftmate details here: http://www.sheldonbrown.com/harris/shiftmate.html
and
Cassette Spacing here: http://www.sheldonbrown.com/k7.html#spacing

According to sheldon, 9sp is 4.34 mm, whereas 10sp is 3.95 mm

Dang, I missed that he was planning to change speeds as well as range. That's not a happy combo.

How about trying a 10spd SRAM cassette and Deore XT RD? The RD should work fine, but not sure about the cogs.

I was thinking about the 9-speed setup to save money on the components. I had thought about a 10 speed setup but the derailleur and cog are more expensive than the 9-speed.

The cost savings won't be so good if I need the shiftmate.

Here's what I did to my '09 Aurora Elite (GREAT bike, btw): install a 26t small chainring up front to replace the stock 30t small ring. Front shifting is still very good. I'm still using a 12 x 27 10 speed cassette.

If I wanted lower gears than that, here's what I would do: install one of the new 10 speed cassettes by Shimano or SRAM in something like 11 x 32 or 11 x 34 and install a Shimano mountain bike rear derailleur, realizing that the new Shimano Dyna-Sys "10 speed" mountain bike derailleurs have a different cable pull ratio than other current Shimano rear derailleurs, so I would need to stay away from them because they won't be compatible with my 10 speed road shifters. Oh, and I'd also need a longer chain if I installed one of those cassettes.