FYI, reports from others who have tried welding cornies (like Yuri)... it's a total bitch because the material is so thin. I'd also be worried about the seam between the cornie and the funnel, trying to get that weld neat so you can keep it clean and sanitized.
Given the small size, I can't imagine the pressure issues that pro brewers deal with would come into play to any appreciable effect. Their yeast is dealing with the pressure of many bbls' worth of beer on top of them, not a couple gallons. I can't imagine the effect would be appreciable different from fermenting in a carboy (assuming you still had the cornies airlocked so pressure wasn't building up).