i will try to make as much sense of this as i can for you and also try not to be long winded. first off the 2 materials, even though both alum. are very different. the cast is hot poured as you would steel in a liquid form. the tube section is cold drawn in this it means that the grain structure is different. the tube has a dirrection of grain and the cast does not. the cast also has the possibility of having pits in it under the surface that are minor defects in the process. nothing you can do about that. cold formed (tube or forged billet) is allways going to get a better shine due to the material being denser. as for your process it sound good. the possibility is that the 2 different pieces may never match.
when i have done cast in the past i have had the same problems. keep watching this thread and others will chime in and give some good advise. also give a look at the top of the polishing forum at the sticky on polishing motorcycle frames 101 and look at syco's frame. maybe you can get a idea as to what to expect there. hope this helps a little bit and best of luck bro.
cliff