yes. The faster you are, and that speed varies from individual to individual the more separation you want between base bar and elbow pads. You probably want 40-50mm. The slower you are the less separation you want and the larger of an aerodynamically shaped headtube.

My educated guess is that speed breakpoint in somewhere around 40-48 kph and will vary for each person based on the conditions of the day

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Brian Stover USAT LII
Accelerate3 Coaching
Insta

If you fit spacers under the stem you’re also moving the whole basebar nearer to you and shortening reach due to the approx 72-74 degree angle of the steerer.
Spacers under extensions your moving up vertically so no shortening of reach.