Above.
Is it really that important? If so, what to look out for?
Theoretically it matters, but since the constant and rapid escalation of speeds and other attributes at various positions leads to the availability of superior players not boosted by your theme team(s), maintaining a +1 SPD boost is resource-intensive. To compete in H2H, you’ll settle on perhaps one group of 20 players with a common chemistry, and take the best players available at the starting positions otherwise.
It was different when you could gain +2 SPD with a tenth-tier chemistry in MUT 24. As predicted by some back in August, a lack of available boosts simply made things harder for NMS players.
IMO Theme Teams have never been about being competitive. It’s about being able to build up your favorite teams across eras and having fun. It also relieves the stress of having to chase every new “meta” card every week because you’re just on the lookout for improvements to your specific team. The boosts made it viable, but not OP.
Clearly EA disagreed and found a way to bastardize the concept into “promo themes” to try to push people towards whatever cards they are dropping that week. They neutered the original “TT” but I’m still running a Vikes/Ravens TT with fillers where I can. The kneecapped boosts sucks but being able to combine both if my 2 teams is kinda nice, they also happen to compliment each other fairly well so it works out well that way.