There is no "need" to upgrade but a bigger speakers will sound a lot better at this listening distance.
As I already wrote in the other thread you almost certainly get not enough direct sound at your listing distance.
Try your KH150 with listening distance of about 1m to 1,5m. Compare the results with your current listening distance. You will most likely like the shorter listening distance better. You can get a similar yet better results for your 2.5m listening distance with a bigger speaker.
Have a look at the following table to get an idea of a good listening distance for a speaker size/ di of a speaker.
www.genelec.com/correct-monitors