"Why would you want to go over +P anyway? especially in an already hot load?"
A fair query.
The short answer: 'Cause.
The long answer:
I can understand the manufacturer's position but it'd be helpful if "over +p" was spelled out clearly.
SAAMI only has an over pressure rating system for five cartridges. That'd be the 9x19mm, .38 Special, .45 ACP, .45 Colt and .257 Roberts. The .38 ACP would be in that category, but they changed the name of the real hot loads to .38 Super back in the 1930's instead of adding a +p to it. These are all because the metallurgy of firearms improved, allowing new guns to handle higher pressure in a respective cartridge, but someone wanted a warning system to keep a real hot load from being chambered in an old gun.
Ammunition manufacturers will throw a +p or +p+ designation on some of their offerings even if SAAMI doesn't have any specifications for such. Either it's marketing or they just don't wish to have someone find out the ambulance-y way that that unsupported match chamber coupled with a polygonal rifled barrel aren't a great combination for the hot stuff.
There's no standardized over pressure system for the 10mm. Informally it's usually divided up into four categories. They are:
FBI Lite - 180 grain bullet traveling at 1,000 fps for around 450 ft-lbs
Moderate - 180 grain bullet traveling at 1,150 fps for around 550 ft-lbs
Full Power - 180 grain bullet traveling at 1,275 fps for around 650 ft-lbs
Nuclear - 180 grain bullet traveling at 1325 fps for around 700 ft-lbs.
These are just guidelines; you can see there's a gap twixt each level and giving or taking 25 to 50 fps likely won't cause the world to end. Let's also assume that these numbers would be produced by a 4.5" or 5" barrel on a 75 degree day. Let's also keep in mind that velocity doesn't always correspond to pressure; change the powder, case, primer, seating depth or some other variable and you can increase the pressure, often drastically, without noticeably increasing velocity, so velocity itself is just a guideline when it comes down to it. That being said...
I'd be hesitant to push anything over 1,375 fps or so, and more nervous about life as I crept up on that. Under 1,335 fps or thereabouts I wouldn't worry at all, provided I had a supported chamber and new or once-fired brass. The straight blowback situation would make me skittish however. Perhaps unwarranted but I'd be skittish none the less.
It's possible that some cartridge manufacturers regard the Nuclear or perhaps even the Full Power loads listed above as +p+. It wouldn't surprise me if a firearms manufacturer thought it prudent to disclaim using +p ammo in their guns, just for liability's sake. A Full Power or Nuclear load won't explosively disassemble a modern firearm in good working order, but it will increase wear and possibly accelerate warranty work.
It's always worth noting that many ammunition manufacturers water down their 10mm offerings so to type. Not just their economy range fodder either; Federal's Premium Personal Defense 180 grain Hydra-Shok JHP shows 1030 fps from the muzzle. That's a whopping 40 fps speedier than their equivalent .40 S&W load. Contrast that with Buffalo Bore's 180 grain 10mm load at an advertised 1350. Buffalo Bore states that this load is operating within SAAMI specs, so it's not +p (to them at least).
Accordingly, at least a few firearms makers ship their pistols under sprung. The spring weight is fine for the economy or watered down loads, but if Full Power loads are used the pistol will get beat up in a hurry. It's occurred to me that it'd be relatively inexpensive and go a long way towards a company's PR if they'd just include a spare recoil spring or two with a note about increasing the weight for more powerful ammo. Sadly, it hasn't occurred to the firearms companies. Instead, some advise not to use +p loads, whatever that may mean.
I was a bit rushed in the post where Billl left the comment, so I glossed over the +p warning instead of mentioning anything about it. Without knowing what the manufacturer regards as +p in 10mm, then I don't know what should be avoided let alone why. So going over +p in a 10mm may mean pushing a 180 grain bullet past 1400 fps or it may mean lobbing it along at a leisurely 1200 fps. If I were serious about buying their product, I'd contact them to clarify that first.
The loads I've worked up for my guns run around 1235 fps from a 4.75" barrel at or just below freezing (so I assume it'd be closer to 1300 on an 80 degree day). That's for locked breech pistols though; a straight blowback may be fine with that load but "may" isn't a lot to go on when there's over 37,000 PSI within grenade distance.
Why go over +p in 10mm? Cause if I'm going to have to promise my first born for a box of ammo (have you priced 10mm lately?? Scrooge wept!) I at least want it to go 100 MPH faster than a .40 Short & Weak! And to some folks, 150 fps may be past what they consider +p.
Now, since I've got the song stuck in my head, y'all enjoy :)