JK,
I am just a beginner at this stuff, but I did do some upgrades recently and researched things of this nature. I am also not familiar with Dura Ace 8-speed. However, the Dura Ace 9-speed is rated for 26 teeth max. But if your friend is using a 28, who knows. Would he let you borrow his cassette, just to put it on to make sure there is not something different about your setup that would make it unworkable? Just ride around the block to make sure it shifts reliably. As far as using a mtb cassette, my LBS told me that you can use pretty much any Shimano cassette - HG70 (105), Ultegra, Dura Ace, as well as the mtb cassettes - with any group as long as it's all for the same gearing (8-speed, 9-speed, etc.) and type - HG, IG. Same for derailleurs. In fact, I am using an XT rear der with my otherwise 105 group, 9-speed, STI, etc. Purrs like a kitten and gives me capacity up to 32 if I want to use it. If you look back through the old posts back around Feb or so, I asked here about using the XT rear der. One of the pros wrote that they set up road bikes all the time with XT for the very purpose of riding in the mountains.
Nashbar.com has Deore XT rear der on sale for $40. I know it is a few steps down from your DA, but it will give you 34T capacity in case you do want to go to more than 28. You could just swap back to your DA when you don't need the capacity. Nashbar also has some HG 8-speed cassettes for $24.95, but the only choices in your chosen range are 13-26 and 11-30.
Somebody correct me if I have gone astray here.
Regards,
Raymond
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