It has been my experience that in a 3D plane, anything over 2:1 thrust to weight is a waste. With 2:1, you will punch out of about anything pretty fast. If you can get more without a weight penalty, go for it, but that is often difficult. You can get light weight power plants in about any size, but if you go too big you end up adding weight to reinforce the airframe. There come a point where more power is not not better because of the side effects of supporting that power in the airframe.

Man, my brain doesn't work well in metric. From my calcs, were talking a total weight of 2.7Kilo with engine, or about 6 pounds total. If the Saito 91 can turn a 15x8 (in inches) prop at 8000 RPM (just guessing), that is a thrust of about 10 lbs, or 4.5 kilo. That is 1.7:1 or close to the 2:1 ratio. I'd say you would be set up just fine with that combo.

Comparable plane? My 2M pattern ship weight 10.5 lbs (4.8K) and I am spinnig a 17x10 at 9200 with a YS140DZ, that equates to 17 lbs of thrust, or a 1.7:1 ratio. My 35% carden has a ratio of 2.1:1. I have had ~2 meter fun flys at 2.2:1. Had a 40 sized fun fly a little smaller than your setup with a 2.5:1 ratio. They all fly well at these power to weight loadings.

Your concerns about airframe are probably justified. Softmounts go a long way to reducing stress. I guess just use your best judgment in deciding if the airframe needs to be reinforced to handle the engine. Might also check the tail feathers as vibration can often weaken them.

Probably won't need to kill the engine for landings. Just keep your prop diameter large and pitch low and you will slow down just fine.