Just got it and tested it on multiple people. Level 1 only gives 5 more def than Protect IV.
.....
Capped enhancing, regular setup def for me: 240
Protect IV: 295
Protect V: 300
dotdotdot... and it seems the progression is two more def for each merit, so it'll cap out at 15 more than Protect IV? I'm not sure how worth it that is. Shell V looks to be an even worse progression because the help text says it gives 1 more magic def bonus for each level.
We can only hope the formula is wrong, but ... I dunno. Feels like I just wasted 3 merits, unless I'm looking at it wrong.