I have 2 STI 170mm mags that will hold a reloadable 28 rounds without the Arredondo spacer. You really shouldn't need it for 28 rounds. Has the tube been tuned at all? I would be reluctant to try and hammer on it with a punch to try and extend the ribs. I thought about this once before but I think it would be difficult to make it very smooth and I wonder if the follower would still get hung up if the transition is not smooth. Also, as you're hammering, you may compress the tube if it's not supported from the inside. SVI tubes are not necessarily 'easy' either. The new style tubes are different and require some work to get them to run in an STI frame, assuming you are using an STI frame here. You can search the forum and find some information on what's required and difficult people have had with them. Not to say they can't be made to work...just require some effort.
Sorry, I don't know the history but, for the STI tube you've got, are you using Gramms spring/follower and Dawson basepad? That's the setup I have in both of mine and they hold 28 rounds.