Hi Ali and welcome to UKAPS.
i have recently bought a Vision 450 with the new High-Lite T5 lighting having upgraded from a Rio 180 with T8's, i have added an additional two 54W T5 arcadia plant pro tubes on a Glo twin ballast and the arcadia seems a little brighter. the difficulty is that the juwel tubes are the "day - 9000K" and "Nature - 4100K" tubes, where as the arcadia is more purple, i suppose around 8000K and there are two of them so it's hard to see which is brighter. i can say for certain that the juwel T5's are noticeably brighter than the T8's. As to plant growth i dont think there will be a whole lot in it but visually the plant pro tubes are a bit too purple for my taste on their own. when all four tubes are on together it looks much more natural.
sorry if that's just confused you more lol
your other option which i would do in your situation is buy a twin ballast such as the glo one and add two new T5's to your existing T8 setup.
if you look in the picture below these are my tanks the first a rio 180 and the second the Vision 450. what you do is thread the end caps from the cable cut-outs at the back of the tank through so that on each side one is in the gap behind the original tubes and the other is threaded under the light bar and out in front of the original tubes. then attach tubes and reflectors. overlap the reflectors from the juwel tubes with the new ones and pull the cables tight at the rear. then fold up a good sized bit of tin foil to sit on the tanks centre brace under the new tubes (this reduces the heat on the plastic).
just juwel T5
2 juwel and 2 arcadia (front two tubes under one reflector)
notice how the tank bottom is far brighter with the 4 tubes on
the ballast reflectors and tubes
just ask if i wasn't clear enough on any of that.
this will give you pretty much 3 watts per gallon and you will be able to grow just about anything given you have pressurised co2 and a good fert schedule.