When you say '9 HPC' system, are you talking about the system based around the Diamond 9.0 speakers?
If so, I would recommend nothing less than a system based on the 9.1 speakers.
The 9.2 and 9.4 speaker are virtually identical in terms of drivers. Obviously the 9.4 is a floorstanding speaker and isn't going to require a stand. Other than that, the 9.4 is slightly more efficient and his slightly more bass response. For the same price, unless space was a premium, I would go for the 9.4, but I'm odd that way.
Based on my best recall, the 9.0 system is just under £500, the 9.1 system is in the £650 to £700 range, and you can add another £100 to that for 9.2/9.4 in the front.
That is quite a range of prices. Could you give us a clearer idea of what your budget is.
As to the Mission M30i's, the 'M' line has been around for a while and has proven itself; it certainly has it fans. But the M30i is the smallest in the 'M' line and is mostly used for a rear speaker. I typically don't recommend anything less than the M32i for a front speaker.
Comparing the Diamond 9 line to the Mission 'M' line, these are the equivalents -
9.0 = M30i (4", 100mm woofer)
9.1 = M31i (5", 130mm woofer)
9.2 = M32i (6.5", 165mm woofer)
In comparing the two lines, the Wharfedale Diamond have slightly better low frequency specs, and the Mission have a very slight edge in Sensitivity. I would personally consider the Diamonds a slightly better speaker, but that's just opinion.
The speakers are similarly priced until we come to the 9.2/M32i comparison, in which case the Diamond is about £70 more than the equivalent Mission.
So, in my opinion, you would need to go to a system that has 9.1 or 9.2 in the front to consider this an upgrade. Simply switching to Diamond 9.0 would be a step sideways. Whether you would like the 9.0 enough to justify the cost, only you can tell.
Again, give us a budget and your requirements, and we will make suggestions at or near that budget.
Certainly a new system based around the 9.1 or 9.2/9.4 would be an improvement, but whether it would be an improvement to justify the money, again only you can tell.
You could simple replace the front two or front three speakers with something bigger and better. You already have a good sub, and the Mission M30i should make great rear speakers. Maybe you can move the two M30i's that you replace in the front of your system over to your computer (assuming you have an amp to drive them).
To replace just the front speakers and the center with Diamonds, would cost about £325 (9.2/9.CS) or £250 (9.1/9.CS).
The 9.CS speaker is slightly better and slightly larger than the Mission Center speaker you have. It depends on how satisfied you are with you existing Center, if you like it then keep it, otherwise upgrade to the Diamond 9.CS (£125).
I think the Diamonds and Mission are similar enough that you could mix them together with no problems. So to start with, simply upgrade your front left/right and work from there.
Again, it is always best to start by giving use your budget and your requirements.
Steve/bluewizard