My DYS kid with a gap between GAI and processing speed of more than 60 points, who is also dysgraphic and has DCD, is also grade skipped and has an additional two year acceleration in math and science.
Yes, writing can be a challenge and but I'm 100% positive it is a battle we'd be fighting regardless of the grade skip. His handwriting is close to that of a 1st grader and his speed is about that as well, despite being a 10 year old 6th grader. A year of OT and PT did nothing to help that, so he has a 504 with accommodations. As long as the accommodations are in place, all is well.
I throw this out there because sometimes I think the school can focus too much on your child's weakness or what they "can't" do and that becomes the pinnacle of why they're not going to accelerate or challenge a child. My DS's 1st grade assistant (not his personally- the class aide) would not support the acceleration because he couldn't tie his shoes and "all the 2nd graders could". He still can just barely tie his shoes- thank goodness he's not still in 1st grade because of it!
I also love that polarbear says above that it "actually helps to be given higher level classwork while working on remediation of a very basic skill such as written expression." This was exactly what we found as well. Telling him he couldn't handle the academic work because the physical act of writing was too hard- that was disastrous for his self esteem and his perseverance.