Originally Posted by sb88

Pretty much - for example if you had the Shimano Sora 9 speed cs50 12t-25t cassette, and replaced it with the Shimano 105 11 speed cs5800 12t-25t cassette, the extra cogs you gain are a 16t and an 18t - right in the middle!

What it sounds like you need is a 9 speed cassette with a 28t or 30t largest cog if you want a couple easier gears for climbing.

Additionally, your wheels might not fit an 11 speed cassette - the freehub (that the cassette slots onto) might not be wide enough.

For what it's worth, I think 9 speed is the sweet spot - fine enough increments in gear change, with still a 'definite' feel when shifting gear.

Tyres and fit.