I do not believe Miller will turn into an ace working at that velocity, either. Which is, I guess, where the argument turns.
Forbes would be biased if I asked him if Miller could become an ace; of course he would say "yes." But all I asked was about Miller's velocity, and he said he relied mainly on a four-seam fastball for the majority of the season and rarely threw a two-seamer anyways - he'd use the cutter instead.
Every time I saw Miller in person this year, he sat around 94-96. When I saw him in the CWS, he was 94-96. Actual scouting reports (not online articles, actual reports scout's fill out) had him 94-96. I did not see Miller in the beginning (or the ACC tournament), where it seems he was throwing two-seamers at 88-92, as you said, and as these reports confirm. The only times I saw Miller was in the middle of the season and the CWS.
Anyways, since this is about him becoming an "ace," think about this. If you're any kind of coach or development director, and you have a tall lefty who can bring his four-seam from 93-96, topping at 98, or a two-seam from 88-92, which do you think they're going to want him to throw? I have a feeling they're going to tell Miller to work on that four-seamer, and work on him keeping it down in the zone, which is when he gets into trouble and possibly why there would be days he would turn to his two-seamer. If he can throw that four-seamer where he wants, it's a dominant plus-plus pitch. If he can throw the two-seamer where he wants, it's a slightly-above average pitch. That, ability to throw the slider to both sides of the plate, and his change are things I would assume they're going to work on with Miller. Considering he is where he is, I have a feeling he'll probably learn a few things and learn to keep that fastball down in the zone. Like I said earlier, his biggest worry is developing an average change that he can throw and being able to put his slider on both sides of the plate. The fastball control will come with work.
Oh, and last thing. Your point is taken. I appreciate you providing links to actual articles discussing this. Though a few are from the same game, it does prove there were times when Miller went to his two-seamer rather than his four-seamer. I apologize; it was tough for me to actually acknowledge that before when I had a.) seen him, and b.) seen and heard reports completely contradicting what you were claiming.