IMR4227 likes compressed charges. It doesn't really effect the pressure as long as the powder is a slow burner and is tested at that level. I would still drop below that max load and work back to it to be safe. The type of bullet and where the crimp band is will affect whether the load is compressed or not at the same charge weight, as more or less of the bullet will be down in the case. You will definately know if you double charge a case with this powder.
The barrel length is only slightly an issue in relation to a slow burning powder unless you are talking 2-3 inch barrels vs 6" plus. As always, the longer the bbl, the higher the velocity. I think you might lose 30 - 50 fps per inch. The powder should be burnt in the first 3- 4" of barrel.