There is a way to get 1000 without 2 playthroughs.
First, do the speed run. I did it in about 5 hrs. Don't do any of the seduction side missions, races, and don't go for any collectibles. Be sure to go back to your trailer and perform a MANUAL save before and immediately after doing any of the 3 Dreamscapes. This way if you get stuck or take waaay too long you can always reload your save and replay without a time penalty. FYI.. there's no way to create a manual save during the Dreamscapes. IMPORTANT: As soon as you finish the Bytanic Dreamscape, perform a manual save.
Second, After creating your post-Bytanic manual save point, finish the game in under 8 hrs to get that out of the way. Don't save over this entry!
Third, load back up your post-Bytanic manual save and go for the collectibles, romances, races, and any other achievement you didn't get. At this point you can take your time b/c you don't need to worry about the clock. Also you can relax knowing you don't have to replay any previous levels. I don't think you have to play to the very end to get the 100% complete achievement. You should get that during your last collectible, race, and/or romance.