Hi,
First of all it's not really a good idea to rely on higher lighting levels because light only has a positive effect growth rate, not on plant health. In fact, just the opposite. Those who rely on high lighting almost invariably suffer plant health related issues. In any case, you will not be able to detect the reduction of growth rate because it isn't significant. There is a much higher affect on growth simply due to CO2 tweaking or due to nutrient concentration levels. You said it yourself. If you are satisfied with the growth rates why spend more money to get new bulbs?
Have a look at a thread regarding old bulbs. The thread discusses T5, but exactly the same applies to T8.
Two new tubes... | UK Aquatic Plant Society
Cheers,