I fear that the gap with mantis would be definitely too big at lower than 3 speed.
Are you afraid mantis become too good against auroras? I don't think that will be an issue. It would also be easy to see, so if its really the case, we could revert the speed change. 0.2 speed is already really small.
I don't really like a t1 naval nerf to Sera because they have already a big disadvantage : weakest t1 subs.
So I would favor, if still needed, a fobo small speed nerf to give more reaction time to defend against them with bombers / jester / more time to build frigates in order to intercept them. And since fobos are very strong on land, a -0.2 speed nerf would not hurt too much overall balance (they can still dodge, have great DPS, rate of fire, AOE, etc).
I think sera t1 is very good balanced on land. I wouldn't want to touch it. -0.2 won't make it unbalanced, but the change should really help on water, else it's not worth to do it.
And i don't think it would really help. The problem with fobos are the attention they need to get killed, and a speed nerf would make the fobos come a bit later (even tho -0,2 are a few seconds, its barly any help in that matter anyway), they still need the same amount of attention to get countered.
so:
-it would have small, but still negative effects on land maps
-it has a really small effect on watermaps too
-i don't think this effect does actually help with the problem
But we should maybe determinate if there need to be any additional nerf to sera after uef bomber/jester first, before we waste time discussing this.
I'd guess so though, because if the uef bomber gets roughly as good as the cybran bomber is now, uef will be about as good as cybran is now too, and thats not enough.
Alternatives could be a even stronger bomber, or something with the riptide. While cybran gets balanced via the jester, but the possibilities with the jester are very limited too, to not make it OP on land maps.