No, not really. Sakura and Chiyo didn't evade Sasori's attacks because of speed as we've already seen Satetsu attacks move faster than they could run as they had to put up shields instead. It is specifically stated that they did so through Sakura's training at evading in general as a Medic-nin and Chiyo's ability to control her and raise up her evasive skills. Sasori didn't have a particular way to know how to exactly measure Sakura's evasiveness. Which is the whole point for changing tactics into a massive area of effect attack - to eliminate the need to measure their evasiveness.
Once again, I don't see the assumption here. Let me break it down again.
1. We know that Satetsu is controlled by magnetic force for a fact.
2. We know that the stronger magnetic force is, the stronger the repellant property is for a fact.
3. We know that Sasori can change Chakra into Magnetic Force and keep on pumping chakra as long as he has more for a fact.
4. We know that Satetsu is moved via magnetism's repellant property for a fact.
5. We know that Sasori raises the speed of his Satetsu with the repellant force of magnetism for a fact, proven by the DB.
Conclusion? Sasori can raise the speed of his Satetsu as long as he has more Chakra to convert and strenghten his magnetic force. More so, we've seen bigger Satetsu attacks that are faster than the smaller Satetsu attacks already. Notably, the Satetsu Spears [
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] which's speed is highlighted by the fact that they: a) elongated after being launched at full speed, b) had the sfx: whoosh effect to highlight the speed they were travelling at, c) had a clear effect on the atmosphere/space surrounding it - most likely breaking the sound barrier. Same doesn't go for the small Drizzle. [
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] Spears are a lot bigger. [
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] Want to know why the spears were faster? Sasori pumped more magnetic force into them. [
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] Point? Bigger Satetsu weaponry can be faster than smaller ones as long as Sasori uses the appropriate amount of magnetic force.