They are brighter than a T8, but not so sure for a T8 with reflector.
Also they come in Juwel T8 lengths, BUT can't be used with Juwel lighting units as they use electronic ballast.
I suspect they give the same output into the tank as T8 tubes so you can just replace your T8 tubes. However unlike T8, you can't add reflectors to increase light or rotate the reflectors to lower light levels.
In the end when I was looking to replace my T8 unit, spent ages looking at LED replacements, but most LED's actually produced considerable less light than a T8 tube but more focussed, so might have been better as all the light is directed down. In the end I just went for a replacement T5 unit, expensive I know, but a known quantity of light, clearly making my tank, no questions asked, a "high light" tank, thus I can maintain it accordingly.