Update:
I'm a full time developer now but I graduated with a design degree. It took me two years of full time development to become a competent developer. Mostly because when I started out I didn't know what I didn't know.
From time to time I pick up design gigs, but I'm a mediocre designer now, mainly because my skills have atrophied. I obviously have a leg up on my dev colleagues, but that's not saying much! However if I had a startup I would definitely hire a pro designer. They're not really that expensive, think of it this way; you're a developer your time is worth, what $100-150/hr? Spend your time developing and pay a designer $50/hr.