Setup for the Alps

Hi,

I've currently got a lightweight carbon bike (Cervelo R2.5) that I'm going to use in the French Alps and the Etape. Currently setup with 53-39 at the front and a 12-25 on the rear (Dura-ace 9 speed), which is OK for everything around Brisbane.

Rather than go to the expense of a triple or compact cranks, I thought I may just switch the rear Derailleur to a Shimno XT long cage and use a 12-32 mountain bike cassette. I've already got these, so all I would need is a new longer chain.

Is anyone aware of any problems with doing this? It seems to me I would get essentially the same benefits as compact cranks, with the only downside being slightly slower shifting.

I have a mate who does this for big climbs. Works fine for him.