In answer to the OP, it depends on the school, the curriculum and the child's personality.
Being bored for shortish periods is fine. Being frustratingly, mind-numbingly bored for long periods (6+ hours a day, 39 weeks a year) is not great, as anyone who's ever had a temp job as a switchboard operator or data-entry clerk will testify.
In dd1's case, she channelled her energy into playing mind-games with the teachers, in an attempt to see how rude she could be without saying or doing anything that the teachers could take action on. The result, predictably enough, was that all the teachers hated her and she came home in a royal sulk day after flipping day. She'd pull pointless stunts to get herself sent out of the classroom. We'd got letters home complaining about her attitude, and had to endure her endless diatribes about how much she hated school and how stupid it all was. In the really bad years it poisoned the atmosphere of our whole family life during term-time.
Half a term into her secondary school and she was transformed into little Miss Goody Two-Shoes, collecting A- grades, commendations and praise postcards, and the teachers falling over themselves with syrupy comments at parents' evening.
With hindsight, we should have moved schools at primary level to somewhere that was more open-minded, more creative and original, and less hell-bent on getting as many as possible to Level 4 in Y6. Hindsight is a fine thing, but I don't think I'd let it go for that long if I was in the same situation again. It's not just about G&T, or doing extra work at home, it's more about making sure the child is in an environment where he/she feels understood and valued. If the child's interests and abilities are massively out of synch with what the school is able or willing to provide, then that is likely to cause problems.