I'm slowly getting used to them on the turbo. When I started my heart rate shot up when I went down on them, but that's stabilised now. I put all my spacers back under the stem when I put them on, and have slowly removed them over the last few weeks, so I now only have one underneath - which is going to stay there for the time being as it's more comfortable than not having.
By the time IMUK comes round, you'll be well practiced on descending those hills as you live round the corner so you'll be going full speed and overtaking people as your local knowledge comes into play. Comfort also definitely comes into it, there are only so many position you can be in on a bike, and it give you a couple of extra ones to use when you get bored of hands on hoods, drops, middle of bars ...