Don't worry, it'll be fine. On of my first crit-bikes was one of those fat-tube Cannondales; super-stiff for crits, but well pound you and give bruises on road-races.
Anyway, upgraded from 7-spd to 8-spd when Shimano first came out with it. No problems spreading the drop-outs 2mm per side. The Shimano freehub actually had a beveled lock-nut for this exact purpose; to spread the dropouts as you push the wheel in. You can get these locknuts from any bike shop and upgrade most rear-wheels that use a 10x1mm axle.