How much does line speed impact the turnover of your fly compared to the line weight?

Let's say you have two identical rods, one fast and one medium in a 4 wt. Will the fast rod which generates higher line speeds easier turn over a weighted nymph? Or would a step up in line weight (using a 5wt. line on a 4wt. rod) have a bigger impact over all?

Is there a simple answer to this one?
In genereal terms a heavier line will allow casting of a heavier fly or a more wind resistant fly. You can only do so much with line speed, ie, a 4# line is unlikely to cast heavy leaded size 8 long shank fly far, no matter how much you try and generate line speed! If your talking s 16 gold heads then line speed may be enough.
Also the rod needs to match what you are trying to achieve. No point trying to speed up casting with a sloppy slow action rod.So yes, the faster action rod is the one for the job.
Some reluctane for the rod to turn over the leader may be overcome with a little 'haul' (increasing line speed) on the final delivery, or just shorten the leader if possible.

If you can get hold of a copy of Charles Ritz's book it will give you heaps of information about line speed (and casting). he advocated High Speed and High Line action. I adopted it early on in my fishing and people tell me I am a purist caster. Works fine for me. So get some speed into the back cast and get the line up high.

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