I'll do an SS later this evening. It should be easy to replicate. Wear PVE gear in the L50 region and have 'zero-expertise'.
I recall the Dev's said not to mix / match PVE / PVP gear; and this still seems to be true. I'm sure other PVE'rs are finding the same thing 2018 Expertise. My health is probably around 24K, though remember I'm nowhere near geared.
I upgraded to a 25 purple barrel, it added about 96 pts of damage and didn't effect my 2018.
Here's the advantage for capped out PVE, is you can focus on other stats, primaries and secondaries. I don't know how my damage compares to others (L52 and undergeared but bolstered), FWIW:
Primary Damage: 944-1398
PVP Boost: 60%
PVP Damage Reduc: 37.5%
PVP Healing: 20.83%
These are my stats using two expertise +41 barrels as my only experrtise
PVP Boost: 51.08%
PVP Damage Reduc: 33.81%
PVP Healing: 17.35%
It's a big difference. Maybe with full PVP gear it'll improve but the main stats and secondaries would have to exceed PVE gear, if I stay capped at 2018?
Like I thought... PVP gear has HP around 30k and better damage ratings.
Besides that, if you are in OW with no PVP gear, good luck....