There's a lot of talk in another thread, including some dev input, regarding the merits of the changes to Story Mode mission XP rewards in this morning's 1.21a patch. However from what I saw there don't seem to be many reports of actual leveling results so I thought it might be useful to have a new thread. I also think it might be helpful to separate the objective, factual results from the subjective impressions based on those results so I'm going to try to do that and I hope others will make some effort along those lines as well.
Summary: Based on my experience this morning and afternoon it is absolutely possible for a player with 200% synergy XP bonus to level 1-60 exclusively in Story Mode, without having to do any LQs and without having to do any of the "new" side missions (the ones that award Story Bonus Reward Boxes).
I hit level 60 defeating Doom at the end of Chapter 8.
Details: I have 200% synergy bonus and was using the 10% Cyclops synergy bonus. I did not use any consumable XP boosts. I was leveling Rocket Racoon with a hybrid summoner spec so I did put many of my 9000+ Omegas into +Int nodes in the early 30s.
I avoided traveling through shared zones as much as possible, and where it was necessary to travel through a shared zone I defeated only a minimum number of mobs. I decided to mostly exclude this category of defeat XP from this test because it will be highly variable based on the number and play style of other players in the same zone at any given time.
In instanced areas I fought and defeated most mobs I encountered on the way to mission targets. I did not go out of my way to explore or clear these areas, but I didn't just run past the mobs I encountered. In smaller areas such as the Abandoned Subway in Chapter 1 and the Taskmaster Institute in Chapter 2 I defeated all or almost all of the mobs I saw. In larger areas such as the Brood Caverns in Chapter 7 and the AIM Weapons Facility in Chapter 8 I tried to keep moving at a reasonable pace, defeating most mobs but undoubtedly leaving some undefeated in my wake. By the time I reached Castle Doom I was level 57 and I suspected I would not need further general defeat XP to reach 60 so I switch to a rush play style, running past most mobs, stopping only to defeat mission targets and any unfortunate random mobs who got in the way of said targets.
I completed all Story Mode missions except Mysterious Old Video in Chapter 2 (because I was keeping shared zone fighting to a minimum and thus didn't get the old video tape loot item) and the previously mentioned side missions that award Story Bonus Reward Boxes. At first I tried skipping some of the missions but I quickly found I was going to fall behind the Chapter minimum level curve so I went back and did the ones I skipped. Because I did this in a somewhat random order I'm not including what levels I was as I finished the various Chapters. To be useful I feel that information would need to be generated from a more disciplined leveling run.
Subjective Thoughts: Given that I hit 60 without any of the XP rewards from Chapters 9 or 10, this clearly was not an optimal leveling process. It took somewhere between three and four hours (unfortunately I didn't keep good track of the time and I had to step away from the computer several times during the process, thus the guesstimate). I have somewhat mixed feelings about the time required. It's obviously worlds better than what we had a couple of years ago, and I appreciate that it's possible to level without having to break the immersion of Story Mode by running (and re-rolling) LQs or by suffering through the tremendous frustrations associated with many of the newer side missions. On the other hand this was somewhere between a 50% and 100% increase in time required compared to the common pre-DCS routine (rushes to a subset of Story Mode bosses with LQs to fill in where necessary). I'll reserve final judgement until further testing determines what can be cut out of the process I used this time in order to reduce the amount of time required to reach the end of Chapter 8 while still meeting the zone minimum level curve along the way and heading into Chapter 9 at an appropriate level to take advantage of the XP rewards there and in the first half of Chapter 10.
For my next test I think I'm going to be a little less diligent about defeating mobs on the way to bosses and also try eliminating the majority of Chapter 7 because I just personally don't enjoy navigating through large cave zones stuffed full of annoying mobs.