Nan, by all accounts, the CLP 990 is pretty good, but while in the Yamaha store some months ago, I still have yet to play a Clavinova that feels authentic.
When you outgrow your digital depends on how sensitive you are as a pianist and how good a teacher you have. You might come to a point where you can play the notes and dynamics properly and at the correct speed but when compared to a recording, your playing (on an acoustic, of course) just does not sound as good in terms of tone and colour. Although playing the piano seems to be just about hitting the correct notes hard or lightly, *how* the finger contacts the key and which muscles (finger/hand/arm/body) are applying the force will produce different tones or colour. You might be able to tell such subtleties, but then again, I know many advanced pianists who cannot appreciate, much less produce, the full spectrum of the tonal palette. Merely being able to play difficult pieces correctly at speed with the right dynamics is not enough; this is what separates the mediocre from the good. You can think of this more easily when you compare violinists: the tone produced by a concert violinist is far fuller and rounder and more satisfying tonally than the screeches emitted by a fledgling student. In the piano, the constrast is less dramatic, but it is there if you care to listen.
As I understand it, the sound of a digital is sampled at discreet levels of dynamics (loudness) and everything is interpolated or extrapolated based on these samples. On an acoustic, the same level of loudness can be produced by striking the key in different ways, but the tonal colour can be very different. I very much doubt that manufacturers would produce a robotic arm capable of varying the touch of striking the key and sample a single key in the many possible ways. It is technologically feasible, but the economics of such an undertaking would many times more than what is currently practised. After all, 99.99% of people who buy a digital would be unable to distingush such nuances. Even many pianists who play advanced pieces are lacking in this area.
Therefore, unless you have a well-regulated decent acoustic piano, you would be unable to experiment with your touch and the shifts in tonal colour that are produced. A good teacher should also be able to point out problems with your tone production and teach you how to vary your touch for different tones. However, these may be quite rare; I had the misfortune of spending most of my formative years on the piano with 5 different teachers who never brought up the subject of how weak my tone was. One of these teachers turned out a few international concert artists (don't ask me how) and one of them has a PhD in piano, but none were concerned about tone production. It was frustrating because however well I played the notes, I could never match the sound in recordings, but with my current teacher, I can
This is why people who are considering a digital need to know their goals in playing the piano, whether as a serious classical pianist or an amateur closet pop player. A good digital is fine for starting out when one is more concerned with reading the notes and finding the keys etc., but why not develop the proper technique for producing beautiful tones right from the outset?