My 2 cents here...
Timing should be left stock for more bottom end power unless the Blaster has a huge bore with a terrible dome, which it doesn't in either case. The advanced timing is to allow your combustion to reach peak pressure at the right time, which is never at top dead center, it'll always be after. Since your AFM burns at a static rate (all other things static), it always takes X amount of time to burn, give or take depending on circumstances. If you sacrifice a bit of pressure ATDC so that pressure BTDC drops much more, you'll make a lot of power.
The reason for this is that if you shoot for a bit more pressure on the after side, you're drastically increasing pumping requirements and losing power. If you take any engine and put it on the dyno and advance the timing until it starts to fall off, it will make more power on top than stock. If you re-cut the head to the best design possible and run the same timing, you WILL have to retard the timing to see peak power. With that re-cut head, I'd stay with stock timing.
As for the carb, again, you want to see a dyno. Generally, I like to look at peak torque. Outside of the powerband, a smaller carb will produce more power below peak torque. A larger carb will produce more power above the powerband. In addition, the larger carb will likely push the powerband to the right. Luckily for you, this has been done before. 30-33 range should be about right for you. A 28 has a cross section area of 615mm^2. A 30 has a cross section area of 707mm^2.
To maintain the same velocity characteristics in that region, calculate your main jet size increase from stock in percentage then multiply by .5 and then your displacement increase in percentage and then multiply all that by your stock figure for a 28 (615). Using that, I'm coming up with a little over 800, which means a 32mm carb. From there, go up and down for more top end or bottom end, respectively.
Amartin_72, I see what you're saying, but you're looking at this in the wrong light. Even if you're looking at it in a fluid dynamics type of way, you're looking at laminar flow, which is the opposite of everything with a 2 stroke. Everything flows every which of way on every reciprocation. A smaller carb will increase velocity, allowing the reeds to stay open for longer due to momentum (even through increasing pressure in the crankcase) and contributing to a slight supercharging effect.
As Blaaster stated, a reed spacer will give more bottom end power. It will also spread the power around a bit. On the other hand, it will take that 2-stroke hit away just a tad.
