I've never been a fan of major modifications in packs. Non destructive changes like leaving the top lid behind when not needed make sense. Trimming excess strap length strikes me like cutting the handle off your toothbrush: it makes a statement, but doesn't do much in reality. When people start cutting major sections out of a pack and doing major sewing on them, I've wondered why bother. If you want a significantly lighter pack, then I would sell the one that you don't like and get a lighter one.
But why have the Exos 58 in the first place? IMHO, 58 liters is a rather large pack and meant for multi-day trips with the bulk of extra food or bear cans (I use an Exos 46). The design is meant for good weight transfer and features that keep your items organized and easily accessed on the trail (read pockets and straps). You are paying in dollars and weight for comfort and convenience. If you are using the capacity, I would guess you need the weight carrying capacity as well. If not, then it's time to shop. When you're done, you might save a pound or a bit more buying a UL back with an internal frame, but it won't have the weight capacity and organization features of the Exos. Just removing the top pocket would get you a step closer and you can add or remove it when it pleases you.