I have a 30'-tall Kieffer pear, which produces well and is in a semi-shady area. The lushest limb growth is in the lower half, but a bushy area at the top of the tree blocks some sunlight from an important part of my vegetable garden in the late day. Would I (or more precisely, the tree) have a problem in 'topping' the pear tree, maybe at 20-25' up?
Trimming a Kieffer Pear Tree
Experts agree, "Tree topping hurts trees". Please read this article: http://pubs.ext.vt.edu/430/430-458/430-458.html
If your vegetables are getting 6-8 hours of sun, some shade late in the day shouldn't hurt. Actually, they might appreciate the shade on hot days.