For me, replacing the reed cages falls into the "I am willing to spend unlimited amounts of money on my bike" category. There are benefits, but they are expensive, and need to go hand in hand with a lot of other expensive changes to realize them.
I was suggesting using the petals only on your stock cage blocks. The YZ reeds have 3 petals, but the petals are the same size as the RD's, so you can cut off one and throw it away and install the remaining two just like stock. It will be a minor change to the engine - seemingly more valuable to some combinations than to others. Likewise to banshee crossover tubes, enlarged crossover tube reservoirs, and carb/reed spacers. All can effect how the bike drives but I am not sure that any of these changes actually make more power. However if the combination you use gets you better drive off of corners, or makes the bike more pleasant to drive, the benefits are obvious.
In your pic, the left cylinder is the style of early US 250/350's with the early ring copper gasket. Very hard to seal on stock good running engines - impossible to seal on high output motors. However, it is the best style for modified heads because you can machine the head with a step that makes it sit down inside the recess previously used for the copper gasket. It is then a simple matter to cut a groove for an o-ring, and then cut the center for the squish you want and combustion chamber volume. The benefit here is that the interlocking steps force the head to be exactly centered over the cylinder, or more importantly the piston. Your new squish area will be very close to the piston at TDC, (.035"-.040", or 0.9 to 1.0 mm). Because the piston is crowned, and the stock head arrangement is exceptionally sloppy and you can easily install the head considerably off center, the squish can be way too thin on one side and way to wide on the other. The alternative is to machine a centering tool to perfectly center the head on the bore, and install some alignment pins to locate the head. The only downside is that the early heads are thin and prone to cracking, but my opinion is that this is primarily caused by the gasket design and over torquing. If you start with good condition heads you should be fine.
The right cylinder is the later style 250/400 scheme which has flat topped cylinders and uses the gasket you are familiar with. This is a huge improvement over the early style as tightening the head does not bend the head over the gasket causing it to leak and crack. With care and copper gasket spray you can reliably get heads to seal, but this is no match for o-rings. You need to center and pin these heads for optimal performance.
I do not have a picture of altered early heads, but I have an aftermarket RD o-ring head sitting here. It has a good squish and chamber, but has not been fitted to an engine yet and has no alignment pins or other engine specific modifications. This head was made by Ottoco before they sold to HVC. It is a 350 head without the step so it can not be used on a stock 350 cylinder.