I own the head tube ones, and the BB ones, and use the cheapos for the seat tube. There are work arounds for the seat tube. I don't think you will find work arounds for the BB, with a few exceptions like using eccentric BBs with IGHs etc...

So if it were me, as it once was, I would put the money into something other than the seat tube.

As to can you ream out the seat tubes that much, I would guess you can, I ream out heavily when making internal sleeves. My only concern would be if it was tough to turn the reamer, and as previously mentioned, how far down you want to push the whole seat post and whether your reamer reaches. I'm sure it will be fine. Mainly, they sell seat posts in many sizes so I don't really know what you need to resize the tube for. The ones I get are either 27.2 or 26.8, for which seat postss are readily available