I think that it would likely be pretty powerful, but as you stated, it's brain wouldn't likely be able to utilize the Guyver unit to its full potential... in more ways than one.
I've always assumed that the Human beings particular design is what makes the Guyver unit so potent on it. A body with high adaptability, a survival instinct and a brain capable of higher functions. A T-Rex would have lots of power in terms of raw muscle, but it's cold blooded nature and small brain would not be a good mix for the unit.
So while all creatures designed by the Creators were part of a weapons project, they likely wouldn't nearly be as effective with a unit on them, as the specific conditions that make the Guyvers so powerful, are not in the right balance. ( For example, a Unit boosts a hosts strength by 100x... but that is because of power from the waist orb and control medal using the brain of the host to maximize potential. Without the human brain, a T-Rex might on get a tiny boost in comparison, and may not get any of the known weapons).
It's also likely why they were so scared. They had made the ultimate weapon. It's what they intended, but not exactly what they had wanted. They weren't scared of T-Rex's at all. But squishy little men got them running.