My DS has telescoped 2 years into 1, and is now in a mixed-age class doing course work 1 to 3 years ahead of his accelerated grade. So global acceleration is 2 to 4 years, depending on the subject. Where his assessments have numbered grades, he's getting 95%+ with minimal effort (e.g. 10 mins of study before tests, if any).
Much of the work is project based, and allows him to stretch far beyond grade level as his interest and time allow. I'm grateful he has access to that mix.
IMO, the goal is for him to have to struggle periodically with challenges he finds intrinsically rewarding. Curiosity, self-efficacy, and continuous self-improvement are all important goals.
For example, he started learning his second language last year and is now, according to his teacher, around the top of his class among children with 4 years of experience. Last year required real effort, and he had moments of frustration and feeling like he was "behind", and that required gentle conversations about setting small goals and putting in reasonable, sustained effort.
I was pleased to see him build inner resolve and focus on valuing his own progress, irrespective of how others are doing. To me, that's an important life lesson, as is the executive skill of planning and executing on the plan, which so many gifties never have to learn early.
