Reaching level 29 at-least is entirely possible without the xp bonus, with a ported Shepard. I'd used a save file with a level 60 Shepard from ME1 and that gave a XP bonus in the start. After the end game, I was level 29. I did use the wiki to find the side quests though, so that I don't miss out on any of them.
The interrupt system is quite nice and I do hope they keep it for future games.