Given the maximum flash storage of 1 GB (original Yamaha) or 2 GB (Mutec) you can use with the Tyros, I think they should offer piano and drum packages of maybe 200 MB, which would still be much better than the factory sounds and enable you to install several of them instead of only drums or only piano.
there is 54 drumsets in the pack, if you just replace the 10 most used sets, then you have a 200 Mb drumpack... which i guess works for 90% of all styles..
On top of that, i agree with you, the Kurzweil and the Nord stage also have convincing piano sounds with less then 200Mb total samplesize, the fabulous Nord italian grand (faziolli comes in 4 sizes, 182Mb being the latest.. however, it does support damper ressonance and string ressonance... which is not yet in the Tyros. (could be however as Yamaha mastered this technollogy in other instruments)
Maybe for piano players, they should create a stage piano version of the Tyros 5 with all the CP4 piano sounds as extra...
OR even better, throw a Motif in the mix too... Arps and Karma on top of your Tyros styles.. But then who would buy such an instrument? (maybe if it was only like $1000 above the T5, that would wake up some synth and piano fans?
Donny also has a point, how many tyros 5 players use their instruments for solo piano music?