I conducted a pretty involved test on this subject for work over the last year. As others have said, forget about T5. They'd require some sort of retrofit kit or whole new fixtures.
Our test involved replacing 2' T12 lamps and magnetic ballasts with T8 lamps and electronic ballasts. The T12 lamps were the nominal 20W type, and the T8 were the nominal 17W type. On the surface this would seem to indicate a 15% saving. However, the electronic ballasts themselves are more efficient, and you may be able to use a lower ballast factor type to take advantage of increased initial light output, improved lumen maintenance, and the improved fixutre efficiency mentioned by eljefino. We found a total savings of about 39%.
Per some info from Philips and GE, a 4 lamp 4' T12 fixture with a single magnetic ballast can be expected to draw 172W and put out ~6,600 lumens (@60% fixture efficiency) at mean rated life (these numbers vary somewhat depending on exact ballast and lamp type but are useful for illustration). 4 T8 lamps on a "low" (0.77) factor ballast will draw 97W and put out ~6,300 lumens (@75% eff.) Go with a "normal" 0.87 factor ballast and you're at 109W and 6,900 lumens.
So, figure 60-70W savings. At 6 hrs/day, $0.12/kw-hr, you're looking at saving about $17/year. Probably not worth it from a pure dollars and cents perspective until the existing lamps or ballast start failing, but it maybe worth it just to get the less noisy ballast, improve the starting operation, and possibly go with a different color temp and higher CRI (definitely get 800-series lamps).
As to "de-lamping", it would be possible to go down to a 3 lamp setup if you use a "normal+" or high factor ballast, but then you'll have the potential for increased glare and the fixture might look weird. Probably not a good idea in the kitchen.
Definitely worth shopping around for the ballasts. Prices for the high efficiency types (GE UltraMax, Philips Advance, Sylvania Quicktronic) vary wildly from one vendor to another. We got them in volume for
jeff