Amp power, speaker sensitivity, max spl and apparant sould volume are complex discussions. Adding one item doesn't necessarily make a lot more sound!
You have to double your power rating to increase only 3db Spl. So 200w to 300w isn't going to make a lot of difference. Only 1.5db, barely perceptable under peak loads. Which is a bit like going from 8-8.5 on your amp dial...not a lot. Most amp dials are in 3db increments. 3 db is the generally accepted minimum jump in dbs to be noticable. From 200w, you'll need 400w for a noticable increase or 800w for a larger increase. So that's the amp considerations...
Spealer cabs are not equal and some are more effiecient than others. A more efficient cab will often yield more dbs at a low amp power thus offering a higher over all SPL. But, most drivers meet an optimum level of power to spl quite early and they have a max spl (sound pressure level) at a lower wattage than their max wattage handling. So 300w into a single cab where the cab actually maxxes out it's SPL at say 150w isn't going to produce any more dbs for the exta 150w if it's powered by a maxxed out 300w amp. This loss can be recovered or untilised by adding a 2nd cab. They would share the load and combine their max spl's. Often allowing the full power rating across the drivers and keeping the drivers under their max spl / wattage level. This max spl / watt is unique for each model of driver and varies by quite a lot between brands and models.
So what I'm saying is that a 100w of extra amp power isn't going to add a lot of real world SPL across the same cab. But if you do go for the bigger head, get a 2nd cab to share the load. At 300w, there's not a lot of drivers which will actually deliver 300w of sound. Most max out at quite low power levels (I've seen alot of ratings in the 125-150w). Add a 2nd cab and you can use what is being wasted from your existing 200 watter and use the extra 100w of headroom.
So if you go for the 300 watter, seriously consider getting a 2nd cab to make use of the extra headroom. Two matched cabs usually add a further 3db becuase of the reduced load. So you will gain from three areas all combining in a higher SPL.
Not saying you've said anything others haven't, but I notice, as a point of curiosity, that when this matter of what constitutes a "noticeable" increase in loudness is discussed/laid out here, no one seems to include the point that the corresponding increases in wattage and sound power only strictly apply to increases in a single system...
i.e., 2x wattage increase in system A = ~3dB increase in system A
It is not necessarily the case, that because system B has 2x wattage of system A, that system B will be taken (or measured) to be 3dB louder than system A.
Also, with regard to what's noticeable (assuming one system/unit of sound power), double the sound power is double the sound power, and it sounds like double the sound power. Because of the way we tend to (naturally) look at relationships like this, there is an almost automatic "sense," when viewing formulas, that twice as much power should be twice as loud. But when we step back and realize that sound power/loudness is more like an additive (of discrete units) relationship than a multiplicative one, we realize that it's one that we already understand, intuitively.
Doubling the sound power unit of a one person talking, from one person talking to two people talking (+~3dB), is louder, but it's not twice as loud. It's as much louder as you'd expect...and it's noticeable...it's as loud as two people talking instead of one.
Similarly, (imagine) increasing 200 units of power (people yelling "Surprize!") from 200 people yelling to 400 people yelling (+~3dB), would certainly be noticeable. Increasing from 200 people to 300 (+~1.8dB) would also certainly be noticeable. All of this, of course, requires the use of some imagination, but I imagine if someone were to close the eyes, they'd notice the difference between 200 and 250 people (~.96dB).
It'd be really nice if amps had a wattage readout on them.
