The piston looks better in terms of the diameter ratio. You are using a vented pilot design, so I think the ratio you have is probably optimal enough for that type of valve. Keep in mind that the effective sealing diameter may end up being the step of engagement since the pressure builds so fast that significant leakage to the o-ring may not have enough time to take place during combustion. I'm not sure, but it shouldn't matter that much either way. You may just have to experiment with the actual opening pressure and performance versus your calculated opening pressure and performance.
The next issue I see is the bumper. I assume that little rubber washer in the back of the pilot is a bumper? Or is that used as a guide for the rod? It looks like a bumper. It's going to get smashed to hell and it will be as if it wasn't even there. A 10X mix has a theoretical peak pressure of around 1100psi. Multiply that by the front surface area of your piston to get an idea of how much force will be pushing back on the piston. Hint: It's going to be a lot.
Keep in mind that using a cannon at 10X is probably a real pain in the ass in terms of practicality. If you need 200psi in the pilot for every shot, then that's going to suck, right? You either need a high pressure hand pump or a high pressure air compressor. That's one of the main reasons I like close piston ratios, low pilot pressures and lower mixes: practicality. The increased safety is just a bonus around here of course
Another key thing to consider here is the piston travel and minimizing it as much as possible. The piston is basically a projectile and every time you shoot the cannon, you are shooting your cannon with itself. The farther the piston travels, the more time it has to accelerate and that means it will slam harder. I'm not sure how big of a deal that really is because I've only really built one decent piston cannon, so my experience is limited there, but I would try to minimize piston travel as a top priority.
A rule of thumb for piston opening distance is 1/4 of the port diameter. So, if the port is 100mm, then the piston can open 25mm and that will be sufficient, although this is assuming the piston is just a flat face up against the port. My design wasn't exactly like that and neither is yours, so your piston will have to open more than the 1/4 rule would suggest. Basically aim for 1/4 the diameter in terms of clearance from the port opening. Your piston sticks inside the port a little, so it needs to open that much plus the 1/4 rule plus just a bit more for good measure (also the bumper will compress, allowing a bit more opening).
If the total piston travel distance is 25mm, then I would aim to have a bumper that's 25mm thick (or a little more). That's not a rule or anything. It just feels right to me having a bumper the same thickness as the piston travel distance. My piston travels 1/2" and my bumper is 1/2" thick. Keep in mind the rubber will expand outward when it gets smashed. Try to have the rubber almost fill the volume in which it occupies so that the rubber will absorb the impact, compress and then fill the void at which point the rubber should then behave like an incompressible fluid, preventing any further piston travel.
If the rubber isn't thick enough or hard enough and if it's allowed to get smashed too thin, then it will be an inadequate bumper and you might end up cracking your pipe fittings or damaging the piston, or both.
Keep us updated. Good luck.
EDIT: For your reference, you may wonder about rubber hardness (durometer). I use a 50 shore A hardness buna rubber for my bumper and sealing face in my 3X hybrid, and it works perfect. That's only 3X though. You'll have to decide on a hardness based on either your gut, experimentation or both.
Shore Durometers of Common Materials
Material Durometer Scale
Bicycle gel seat 15–30 OO
Chewing gum 20 OO
Sorbothane 30–70 OO
Rubber band 25 A
Door seal 55 A
Automotive tire tread 70 A