Piano models and version 5

I think all the builtin instruments and standard add-ons are already updated for Pianoteq 5, as you said. That is, the D4 and Blüthner and U4 already profit from the better engine (and sound quite different between version 4 and 5). YC5 was due for a major overhaul, apparently. K2 is a completely new design which (in my opinion) has more or less nothing to do with the K1 of Pianoteq 3.

KIViR is another case: the instruments have not been adjusted and updated to make use of new engine features since at least Pianoteq 3, maybe even longer. Since the focus lay on the Kremseqq instruments for the release of version 5, I guess there just wasn't enough time for everything, so KIViR got delayed. But I would not expect version 5 instrument overhauls apart from KIViR at this point.

BTW, a Blüthner "Model 2" would be a completely different piano from the Model 1. The Blüthner Model 1 is a real instrument, manufactured by the Julius Blüthner manufactory in Leipzig, Germany (currently one of the oldest piano makers in the world). It is a 2.80m (9'2") concert grand in the same league as the Steinway D. Blüthner grands have a characteristic mellow sound and still are very clear and brillant in the treble register due to their unique 'aliquot' strings (which are quite different from the normal duplex scaling most other grand pianos use). The 'Model 2' is a somewhat smaller concert grand than the Model 1, with 2.38m (7'8") length.