Steaks taste horrid in the center? I followed in this answer to How do you properly cook a steak?,
but I ended up with one problem. After cooking in the oven for 10 minutes, my steaks were soggy and flavorless in the center, yet okay on the outside. I cooked a few minutes longer in the oven and the steaks started turning white and firm, obviously overcooked.

Should I flatten the steaks first; is thickness the problem? I had 1-1.5 inch thick ribeyes from Whole Foods.

One thing: after searing the steak in the pan, my oven wasn't fully heated yet. So I stuck the steak in at around 267° F and it sat there for about 5-7 minutes before reaching 425° F, after which I cooked it for 10 minutes. Could this have messed it up?
If your steaks were soggy and flavorless then the steak wasn't good to begin with. It's possible it was old as well, or had been frozen and thawed. It doesn't sound like your method was wrong (except not having your oven to temperature but that shouldn't make them soggy and flavorless), but meat itself wasn't good. You paid $6 per steak, I'd expect to pay triple that for decent meat to be honest.