Cycle Sportives

you reckon I'll need a 27 even with a compact?

Tbh - I'll need all the help I can get for the L'alpe D'Huez climb at the end

I presume you're doing the etape? Depends on how much of this sort of climbing you've done in the past. Like Hippy said, not so much the steepness but the sheer duration that gets you. By the time you get to Plan Lachat on the Galibier you will (apart from a short descent into Valloire) have been cycling uphill for a very long time and then you hit the top of the valley and the road goes up the valley wall to the pass itself, some bits at that point are at 10%ish and you're now at 2,500m above sea level so you get a little bit of altitude effect as well. Oh, and it could be cool as well, last year on the Marmotte the top of Galibier was 16 degrees and overcast, Bourg D'Oisans was over 35 degrees and brilliant sunshine. But then you get the descent, especially after you join up with the Lauteret - it is awesome (except one or two of the tunnels!). A 34x25 on the Alpe at 80rpm avg cadence will get you from bottom to top in under an hour - you need to judge for yourself whether you think you will have that in you at the end of a long day in the saddle (that would be some ride). The 27 was my friend at the end the Marmotte, and I had a compact. There's nothing macho about small cogs and standard doubles when you're walking your bike up!