I had a post about the loot changes including the emperor’s gift with some further discussion about the topic (Emperor’s gift).
Emperor’s gift on 5+ is mainly for collecting blessings (t3, t4) with a good “pre-crafted” weapon as chance. Worst case scenario you get around 1/3 map worth of credits if you sell the item in question.
Based on my experience tomes have far less of an impact on Gift than grims (even a single grim have a noticeable impact, while tomes don’t really). I also found that modifiers have minimal or no impact on item base rating (instead they impact perk/blessing level).
If you get a very good weapon out of it (t3 perk + blessing with around 360+ rating with good distribution) you get around 10-30 games worth of resources (that would be required to get it from the crafting system).
If you are at the point where you are running a 370 bolter with a t3+t4 perk and t3+t4 blessing you are basically at “endgame gear” and getting an upgrade will be rarer, and only a marginal improvement on performance (outside some very few edge cases that I would put in the balance mistake category).