I guess until we know more about how Specialization will work, and what benefits it will give, it's hard to balance evasion chances for different ship types : but as we already have 5 categories for basic speed I might suggest the following :
If we take a 20% evasion chance for a medium speed fleet (defined as the slowest vessel in that fleet) as the baseline ; then the speed of the attacking fleet will either increase or decrease that 20% chance accordingly.
So : based on attack fleet speed the bonus would be :
very slow fleet : evasion chance = 30%
slow fleet : evasion chance = 25%
medium fleet : evasion chance = 20%
fast fleet : evasion chance = 15%
very fast fleet : evasion chance = 10%
then to reflect the defenders speed, they also get the same bonus from their category ; so, a very fast attack on a very fast defender would give both a 10% bonus which would nullify each others advantage, giving the base 20% evasion chance. A very fast attack on a medium speed defender would give the attacker a 10% bonus, reducing the evasion chance to 10%, whilst a medium speed attack on a very fast defender would increase evasion chances by their 10% bonus to 30%.
Where this falls down slightly is that a very fast attack on a very slow defense would end up with 0% evasion chance : realistic maybe, but perhaps a minimum 5% evasion chance should remain, a lucky fog-bank, navigational hazard or other element of chance saves their day.