I need to put on a new cassette with lower gears for riding some steeper climbs in my area (mtns of NC). I'm dealing with an 8 spd Dura Ace STI shifter and deraileur likewise. My friend has used a 28t cog so I know (?) this will work even though DA is speced for 27 max. My question: Can I use an 8 spd Mtn Bike cassette? I like having 11-28 rather than say 13-28. Any essential difference between MTB and road cassettes? Please enlighten me. Thanks.