Having had time to consider this subject, I conclude that with the permission of the other players in your games, you could restore to your Eldar ships their characteristic movement pattern. You would have to provide Engines for both movement turns and you would have to pick one axis of the playing field along which you would only be permitted to travel at half speed (or to put it another way, paying twice the normal movement cost).
It seems that one should multiply the Defense Rating by 2 (for the double movement) less 1/3 (for the third of the playing field on which you would move only at half speed) or, rounding up, *1.7. The effect on the Offensive Rating is more difficult to calculate, but would probably average out to a similar quantity. While the proper way to calculate the Offensive Rating (and find the true Combat Rating for an Eldar ship) would be to calculate the Offensive Rating at half speed, calculate the Offensive Rating at twice speed (doubling this to represent travel along two axes of a hexagonal map), and taking the average by dividing by 3, I feel that for a House Rules game this complexity could be neglected and that multiplying the Combat Rating( as determined from the unmodified DRAT & ORAT) by 1.7 would suffice.
However, given that all ships are counted as inflicting Damage simultaneously in Starmada, being able to make an additional movement after firing would not reduce the Damage inflicted on your ships.Though it would return some of the flavor that these ships had in BFG, it would not help Nomad's problem with them. I regret that I am not able to be of more help to him.