How many watts per gallon do I need with T5?

Usually the convention with flourecents is 2-4watts per gallon, or 20watts
per sq foot surface area.

However for the same length tube (i.e 36in) a T8 flourescent is 25w and a T5
39w.

I have read though that a T5 gives more lumnens per watt than a T8.
(http://www.practicalfishkeeping.co.u...?article_id=75)
If this is correct then when using T5 a lower wattage should be required
than using T8 tubes?

Therefore on a 39inch/29gal(imp) tank (3.3sq foot) what wattage of T5
lighting should be used on a mixed freshwater tank to provide a good level
of lighting for plant growth?

Would a single T5 tube be enough? i.e 39w for 29 imperial gallons =
1.34w/gallon or 11w sq foot?. A single T5 power compact would provide 55w
but fit only in the centre of the tank (i.e 22in tube in a 38in wide tank)
and give 1.89w/imp gallon or 16w sq foot? Alternativly it may just be
possible to fit 2x 39w single T5 tubes which would give 78w or 2.68w/gallon
or 23w sq foot.

If the tube is REALLY 3-4 time more effective than a standard fluorescent
bulb of similar wattage as claimed by pratical fishkeeper then:

if we assume 29 gallon tank and aim for a high target of 4watts/gallon we
would require 116watts of light. If T5 is really 3 times more effective we
would require 1/3 of the wattage which would be 38.6w - or one T5 tube?

Has anyone used T5 in a freshwater tank? is it really more effective than
standard tubes?
Does anyone know if the watts rules still apply to T5? and how much light
(in T5 terms) should I be looking at acheiving? Would a single T5 tube be
enough? or should i be looking at power compact t5 (55w) or even 2xTubes?

Any advice appreciated.