Others have posted some good info on anxiety vs. school performance, so I won't repeat that here. I would like to offer some thoughts on reframing the question you just asked...
"...[H]ow much pressure is too much, and how little pressure is too little?" is a hard question to answer, because it's abstracting a bunch of separate issues that are better addressed, well, seperately:
Requiring that your children set ambitious goals and learn to pursue them, vs. having a more hands-off approach that is fine with your child doing the minimum required at school, and just "hanging out" etc. outside of school.
Making your child feel that his/her family's love and acceptance is dependent on his/her performance at some task or tasks, vs. not caring at all about achievement vs. a middle ground.
Trying to force your child onto the path that you have chosen for him/her vs. wanting him/her to be great at whatever he/she chooses to do.
Teaching your child to accept nothing less than perfection/1st place at everything vs. teaching him/her that failure is part of life and learning, and that you don't have to be the best at something to find it rewarding.
Punishing poor performance vs. a "what happens happens" attitude vs. looking for ways to attack problems together.
I'd say that I was raised in a high expectation but low anxiety environment.
My parents never told me what career path or what kind of relationship/kids, etc. I should want. They wanted me to excel, but made it clear they were equally happy whether I became a farmer, a teacher, a rocket scientist, or whatever. They did everything they could to help me reach my goals, whether it was carting my brother and I off to my speaking engagements, or driving me to the library every day for two weeks because TAOCP was deemed too expensive for a young teen to take home with her while it was on inter-library loan.
When it turned out I was terrible at softball, rather than trying to make me "stick with" something that turned out to be a bad fit, my parents thought it perfectly natural that I chalked it up to experience and switched to volleyball the next season. They treated my oscillating between piano and vocal music the same way. It's hard to know whether something is for you before you've tried it.
When I failed to perform, my mom stuck by me, and helped me seek a solution -- even when everyone else said she was crazy. I became very ill in high school, but none of the doctors we saw could figure out what was wrong with me for 2.5 years. They told my parents that I wasn't really sick, but acting out and lazy. I had teachers trying to get me to drop out of school, and I'm sure the whole thing was embarrassing and disappointing for my parents. I went from the top of my class to barely graduating.
At the end of my senior year of high school, I was diagnosed with a condition that can cause tiredness, immune system problems, nomenal aphasia (inability to recall nouns), disorientation, memory loss, blackouts, migraines, and a bunch of other nasty stuff that had crippled my ability to do much of anything. It was treatable, and life went on -- but it was too late to repair most relationships with those who'd believed that my failures were voluntary.
I've seen a lot of young people pushed into career paths that they aren't passionate about or talented in because of their parents' expectations. I've seen people struggle to be who they aren't out of fear that not doing so will cost their parents' love. At best, it ends in mediocrity and unhappiness, at worst, self destruction, usually in the form of drugs and/or suicide.
The opposite extreme isn't good, either. Kids who grow up with parents who don't care about seeing them achieve tend to underestimate their own abilities, and be extrinsically rather than intrinsically motivated in general.
So, rather than thinking "pressure" is good or bad, I think:
Ambition, goal-setting, unconditional love, self-direction, work ethic, support in overcoming obstacles, and understanding that not everything we try is for life are awesome.
Rigidity, artificially limiting your child's choices, unqualified attitude of "succeed or be punished", and conditional acceptance are bad.