Well, points to points a pair of griffons beats a colossus vs marines.
Assuming that a "hit" from a template causes 6 hits, a colossus would wound 5 times. It pushes all wounds through. However, you'd only hit 1/3 of the time, so on a given shot would cause an average of 1.66 wounds. If you could fire for 6 turns, you'd output about 10 wounds in a game vs MEQ.
Assuming the same 6 hits, a griffon would wound 5 times, and MEQ's save would reduce that to 1.666 wounds, and a griffon "hits" 55% of the time thanks to "Accurate Bombardment", for an average of .926 wounds per turn. However, with two griffons you get two shots per turn, causing 1.852 wounds per turn. If allowed to fire for 6 turns that adds up to 11.112 wounds per game.
That's more than 10% increase in casualties for less than 10% more points. Vs non MEQ, the math gets better, especially if they're in the open. Additionally, the colossus is only likely to cause two pinning checks per game, while the griffons should cause 6.6 pinning checks, if you wound with each hit.
If you can afford an extra 10 pts, you get a much more reliable set of weaponry, with a more useful range of fire. Should the enemy close to less than the Griffon's minimum range, move up with a heavy flamer!
To the point of the original question, the above answers are correct. You need to have an infantry blob kept back to protect an ordinance battery, which is stuck in place and can't fire the turn it moves onto the field, such as in a DoW deployment type. I think the Griffon, for sure, is points efficient as you can kill a squad of marines with them in a game. I try to include at least one Griffon in every game, as I've converted a basilisk by removing the shield, shortening the barrel and removing the underslung sights [?]. If you play a balanced army, including them isn't tough as you'll already have some infantry hanging back as part of your static elements. You might have a tough time including more than a single griffon if you're also including a russ though.
For myself, I always try to fire a pie plate at an infantry squad. With readily available cover, the improved AP of the Basilisk is often negated, in my experience. As such, the Griffon still wounds on 2+, gets an extra "Hit" per game, and is about 60% of the points cost, if I recall [no codex handy].
To the best of my knowledge, a single Griffon can be added to any squadron of "Barrage" tanks. It's Accurate Bombardment rule can be used to help ensure the rest of the weapons are more accurate if you fire it first, as the remaining templates get placed touching the first shot or even on top of it. Think of it as a Pask upgrade for barrage, except it can kill things on it's own.
I haven't tried the other barage tanks, and I haven't run the math on them yet, so I don't have a valuable opinion on them.