Here are some test shots, they are softer with less DOF. But I think they are still usable, mind you its overcast drizzly day so full sun will help alot.
Well my Kenko TC arrived today and the first lens I tried it on was my 90mm Tamron Macro (AF still works).
An example using 90mm, 1.4TC and DCR 250.
And just using 90mm and DCR 250