I probably could use an F30T8 ballast which will overdrive the tube by about 2 watts but would prefer an actual 28w ballast.
No, it won't be 2 W, but it could be well above that.
Plus the electrodes in these modern T5 (HE and HO) lamps do not handle the low frequency well. When their design was developed while optimizing the design of these lamps, the 50Hz ballast compatibility was just "thrown under the bus" in order to get the extra efficacy...