Light vehicles can definitely use Wraithgates. They're very, very useful for letting Hornet formations get the first strike, combined with their native speed and range.
Quote:
Wraithgate: At the start of the game nominate one objective on your half of the table as a Wraithgate before setting up spacecraft and garrisons. The Wraithgate functions both as a Webway Portal and as an objective for the rules purposes. It may not be destroyed. Only formations consisting completely of infantry units, light vehicle units or armoured vehicles units with the Walker ability may use the Wraithgate to enter play
I've argued against having an upgrade cost for Fire Prisms every time it's been raised (they have a very situational advantage over Falcons, but are strictly worse in almost all cases).
As an alternative: what about changing the weapon stats to AT3+/AP3+ and making it a free choice between Falcon and Fire Prism? That's much worse while doubling and firing at vehicles (typical targets for the Lance shot), but a small improvement against troops, and keeps the additional range they bring to the formation. Would that be enough for consideration as an "equivalent" option?
5 point upgrades are useless in the Eldar list: either you're upgrading a whole formation, or you're wasting up to 20 points. You might as well say "+25 points: Any number of Falcons in this formation may be replaced with Fire Prisms."