Like you, I usually wear a relatively stable shoe for general training, but I do have 2 pairs of shoes for faster sessions and road races/triathlons.
I currently have the Saucony Taya - now renamed Team Taya. It's light, but has a large enough medial post to cope with my (mild) pronation.
The other shoe is Reebok 3D Flier - no obvious medial post, but the heel is wide enough to cope. Again, this is discontinued, but www.sportsshoesunlimited.co.uk have a supply at a very reasonanble price. I'm so pleased with this pair, I recently bought another couple and put them away in the loft.
Both shoes are fine for 10Ks and speed sessions - I might risk a half-marathon in the Reeboks, but no further!
Good luck
Paul