Fire Prism being "worth it" is a tricky proposition because it's not just about firepower. For 160 points against T7 3+, a Fire Prism in Focused Mode does: 4 shots -> 8/3 hits -> 16/9 wounds before saves -> 16/9 wounds after saves, -> 32/9 damage, 3.55
For a similar price, you could get 6 Dark Reapers: 6 shots ->4 hits -> 8/3 wounds -> 16/9 wounds after saves ->16/3 damage, 5.33
So, for the same cost, Dark Reapers deal about 50% more damage against vehicles, which seems hugely better. What advantages to Fire Prisms have?
1) Durability. A Fire Prism has about 1.5x-1.75x the wounds of 6 Dark Reapers, depending on how the Dark Reapers are broken up into squads. Also, a Fire Prism has much more Toughness than Dark Reapers.
3) Mobility and positional flexibility. A Fire Prism is not inconvenienced by chargers due to FLY. A Fire Prism can move fast and sacrifice half its shots, or Advance and sacrifice them all. Unlike Dark Reapers, it can't make small positional adjustments without losing accuracy, sadly.
4) Shooting Flexibility.A Fire Prism has more range. Its gun is also better against swarms than Dark Reapers, though a Dark Reaper unit with a Tempest Launcher can mostly match a Fire Prism for this.
5) Linked Fire. A Fire Prism can fire indirectly or with more power using Linked Fire
So, how does a Fire Prism stand up with Linked Fire? Focused mode: 4 shots -> 3.55 hits -> 3.16 wounds before saves ->3.16 wounds after saves, -> 6.32 damage
So, spending a CP can bring a group of Fire Prisms just beyond level of effectiveness of a Dark Reaper unit at tank-hunting. However, this costs CP, and requires everyone shoot the same target. On top of that, Linked Fire has anti-synergy with some Eldar buffs like Doom and The Path of Command, where you're already potentially getting a bonus.
Seems valid and different from Dark Reapers.