T5 are the same wattage for the length (roughly) as T8 as are T10 etc. There is no difference in reality between the wattages and the lengths.
They guys above are talking about T5HO which is more powerful per length and CF also use bent T5HO lamps.
As per the original question the 40W starter may run a 15W tube but it would overload it and gradually burn it out quicker than you want hence the noticeable flicker. If the Rekord actually doesn't flicker it is using a different type of ballast or it is not a noticeable flicker. I would guess the latter.
If you buy T6 tubes (actually T6HO) they will be thinner and avoid some restrike but it will not make the 40% difference that is stated. It says they are 40% more powerful than standard T8 tubes. Not doing the maths here I guess but if a 24" standard T8 is 18W and theT6 is 20W that makes it 11% more powerful. What they don't tell you is that T8HO is 20W anyway
You will gain minimal light from the restrike gain but without a 20W ballast you will still only get 18W wether it be a T8HO or T6HO
So as stated above if you want more power then you need to buy new ballast and tubes to do so.
30W over a 125USG tank sounds tiny to me. Are you sure it only had a 15W tube in it before? or are we talking about a Rekord 60. If so 2 x 15W T8 should be more than enough.
AC