Just remember one thing! Mobile phones are not HI-FI electronic components. Therefore they should be regarded as phones and not SUBWOOWERS.
By changing the original sound drivers you might get into trouble, because other drivers are not signed by Sony Ericsson and on the other hand they are not designed to work with K800 speaker hardware specifications.

You should stay with the original ones even if they are not so loud. Do not try to compare the K series (designed for photography) with the W series (designed for music).

Be well!