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It depends on the T8 and the T5HO fixture, but generally yea, a single T5HO, especially with a reasonably good reflector, should out perform two T8s, especially if they don't have a reflector.
The T5 HO will give you roughly 54 watts and the two T8s would give you a bit more at about 64 watts, 32 watts per bulb. The T8s would be the best, because they will give you more light distribution, but a lower intensity than the T5, which will shine a brighter light, but over a much smaller area.