You are right they are not similar at all: terminator armor slows movement, hey wait so does gravis armor. Terminator had a special bolter not available to space marines at the time that put out twice as many shots, oh wait new gravis armor can carry bolters that shoot 2.5 times as much.

I think boltstorm aggressors are very similar in function and role to tactical terminators.

All they need to do now is have gravis armored troops that give them an ability to go behind enemy lines and have strong melee combat as an equivalent to assault terminators, although they did recently put out relic terminators in 30k. If they do it I could see Dark angels getting an entire box of new gravis deathwing models.