Yogiboy ,
The Harbeth M30.1 are very nice speakers. They have that Harbeth sound with the fantastic midrange.
You would get a bigger soundstage with the 30s compared to the P3 and of course more bass. The M30.1 does have the silk dome tweeter , which is different from the P3 and will have a slightly different presentation. Whether you would prefer the 30s over the P3 would be up to you. It is something you would have to try.
It is always best if you could audition for yourself and make the comparison at a local Harbeth dealer. If you do not have a local dealer , than trying a used pair would be you next best option. You probably could turn around and sell a pair of M30.1 for close to what you paid for them.