Does speed play a role on both offense and defense? Sensible that it would but don't know if anyone has ever collected data to check the impact. I've always used a combination of speed and range when looking at outfield positions, as opposed to just range, but not sure how to weigh the traits relative to one another (especially for CF).

I don't think speed plays a part in defense. That's all about Range. That's my understanding, anyway.

Speed and base running work well together. With both in the 90s my players have stolen over 100 bases for consecutive seasons.

Speed has no bearing on defense.

It is all about range, which is different than speed. A guy could be speedy, but doesn't get a good jump on the ball and vice versa.
Range is also improved by how well a player might read the ball coming off the bat.
Some are much better than others at this and whether or not they are "speedy" that quick break in the right direction really expands their range.