Obviously coupons do give the shopper an extra incentive to make a purchase, but the truth is it doesnt matter whether or not you're a coupon guy or not when it comes to your conversion rates.
what it does depend on tho, is how well you design your site and the type of calls to action you use. it's not enough to be a coupon guy, you have to do your job at selling the coupons so that your visitors are encouraged to use the coupon itself.
trust me, we have MANY coupon guys in our program, and some do really well, while others fizzle. probably due to a number of reasons; some may have better seo, some may have a better design, some may have better Calls to Actions, etc etc. Point is, i dont think there is any merit in being a coupon guy and your conversion ratio. It's all about how much work you put in your site and your ability to give your visitors that extra push to use the coupons you advertise.