I think people forget how long ago the Death of Superman was. It took place in the beginning of 1993, only some 7 years from his Man of Steel reboot where he was pretty significantly depowered.
Compared to the distance from DoS to now, some 13+ years later during which Superman has consistently grown in power more and more, that is to say, Superman gained more power in the last 5 years than he did in the first 15 combined (which makes sense- the 11-15 year olds reading Silver Age Supes and Man of Steel, grow up to become the 30-somethings which control DC and make an effort to return some of those Silver Age aspects they remembered as children).
That was the trend of the era as even Wally/Flash only ran around super-sonic speeds and had a major metabolism handicap that required him to eat constantly to keep up his speed. So compared to today's power-levels, Doomsday outspeeding Flash or overcoming Superman's lesser invulnerability of the time, was fairly plausible. If current Supes faced off against that era's Doomsday, it would have easily gone to Superman.
As for dumping him in space. After tearing away most of Doomsday's green jumpsuit, that was the first thing he tried to do, but Doomsday was too quick and too strong and was able to twist out of Superman's grip...