I dont know that I have ever heard anything about seeding for sugar grain size? Now in cream yes I can see that seeding with different grain seed cream may change the smoothness. Back to sugar. So you tried a batch at lower final boiling temp and its was courser but did not go through the sieve good.
I can only give you one hint and it may just be my luck, type of syrup, boiling point or mixing equipment? But I rarely ever sift my sugar.
i use amber or light syrup, boil to 260F, transfer to Hobart 20 qt mixer and stir on low speed. watch the mix and stop when most of the heat is gone from the bowl and the grain looks right. I dont always win first place with my sugar but its generally nice medium color with medium grain.
Hope that helps.
Regards,
Chris