.020 over is 353, 0.040 is 357, 0.060 is 362, 0.080 is 366. The 366 is the max bore you can do. That being said, it make the cylinder walls thinner and it can become more subseptable to heat. Is a 366 more powerfull than a 357? Not by much. 366 also means that you cannot bore it again for another rebuild, you'd have to either get another cylinder, or have yours resleaved. You'll have to bring it to a machine shop and have them inspect and measure the cylinder, to see if it even needs boring, or if you can get away with 0.020" over. Some cylinder have so much wear that they have to go straight to 0.040" over. You also asked about overall speed, I'm assuming that you mean will it accelerate faster without gearing changes? Yes it will. More power means it can turn the rear wheels faster. Imo, save the money on the 366 kit, and go .020 or .040 over, and use the money you saved on the 366 to buy some port and polish. You money would be better spent on head work at that point.