I take Don Mills every day; it's congested. I haven't taken Sheppard east of Don Mills during rush-hour since the 1980s ... but it was congested between Don Mills and Victoria Park back then! (Don Mills actually runs better than when I took it back in the 1980s ... the bus lanes really help ... but there is still too much traffic for the buses, without bunching, packed buses, etc.).Finch west and Eglinton maybe. But Don Mills, Jane and Sheppard East? Congested? Please. Sheppard East ia not even congested during rush hour past Markham.
I confess I'm not as familiar with Jane in peak, but given that the it has about the same number of passengers as Don Mills, is a shorter route (to Steeles), and is more frequent; I have a hard time believing that it doesn't have similiar issues, and is at capacity.