I thought it had an impact on the speed the GPU could obtain?
In simple terms, to achieve a higher DP rate you have to make the shaders more complex in ways that that won't help SP performance. This isn't the issue for most consumer cards since their DP rate is locked by the manufacturer (the 780 is capable of the same DP rate as the Titan, it's just fused off/disabled in the firmware). The reason why the Titan and 780 are so close, is that at a certain point increasing the clock speed is going to net you the same performance increase as going wider but with less power draw and they're pretty close to that point. That's why people are talking about a full GK110 not being viable, unlocking all the SMX might mean that to hit the 250W TDP they would have to lower the clock speed too much to beat Titan.