Is 11- 34 rear cassette necessary?
For grades beyond 10% having a gear that you can spin at the rate you can climb makes a big difference. 

Only you can know exactly what gear that should be. If you can find a gear ratio tool that displays speeds for a given gear, wheel size and rpm. This one seems pretty good.

http://www.bikecalc.com/cadence_at_speed

Then think about your typical speeds while climbing on the steep sections. You need a gear that will allow you to spin at a least 60 rpm and still keep those speeds. 

A gear that works at 30% and 60 rpm probably isn't possible with a compact crankset. 

FWIW, Where I ride has lots of climbs above 10% in the 5-10k range and I see lot's of riders with compact cranksets and 11-34 clusters. 

Personally, I think if you are really considering this, you'd be much better off getting a compact 110 Bcd triple and something like 24/36/48 gearing. It's not that you get that much lower gears, but you get many more useable gears. Having a choice of a few gears in the low range makes a lot of difference compared to just the one or two extra low gears you get by going to a wide range cluster.