The Current Nova Extreme T5 HO fixtures are generally regarded as lower end hobby fixtures. The original Nova Extremes didn't include individual reflectors, which are key to making T5 HO lighting as useful as it is. The Nova Extreme Pro fixtures attempt to remedy this, but I haven't seen one first hand to really comment on the quality of the reflectors. At the same time, however, I've never been a fan of the Current bulbs that are included in the Nova Extreme / Nova Extreme Pro fixtures, considering there's far better bulbs available that are in the same price range.
That said, a 6 x 54w Nova Extreme will be fine for softies and most LPS in a typical 90g. Many SPS would be ok near the top. But, you may not have great success with higher light corals. A clam would be possible, if kept middle to high on the rock work. If it's a 6 x 54w Nova Extreme Pro, then you'll be a little better off than if it's just the old Nova Extreme model.