It doesn't matter much now because Grier is gone, but by almost all objective measures he did draft better than most GMs.
I've did a pretty extensive statistical analysis of GM draft performance over the last 10 years around the time Grier was fired to see which guys have actually been the most successful drafters. For certain guys that I have potential interest in as a GM candidate for the Dolphins, I extended the analysis over their full careers.
There are 60 GMs in the analysis, but the screenshots below may not show all as they just show what fit on my screen when taking the image. I took draft results from Pro Football Reference and put them in a spreadsheet to be able to compare them across a variety of measures.
The measure that I think it probably the best of them is career Weighted Approximate Value/Player Year Per Average Pick Value. Weighted Approximate Value (wAV) is used to determine a relative value of all players at all positions. On its own it's far from definitive in that regard, but it's still a useful and objective metric to use. The Average Pick Value is the average value of each GM's draft slots based on the traditional JJ trade value chart. There are things one can argue about there too, but it is a commonly used and referenced measure of the value of picking in various places in the draft. This levels the playing field for GMs who have tended to pick later in the draft, i.e., with less draft capital.
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Grier is 31/80 in this measure. Packers GM Gutekunst is 13/80.
Below is sorted by wAV/Pick, but I don't think that's as useful because it doesn't account for the relative value of the picks.
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On this one Grier is 22nd and Gutekunst is 39th.
Below is All Pro Seasons per year drafting:
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Grier is 17th. Gutekunst is off the charts as he has not drafted an All Pro.
Below is % of picks that made at least one Pro Bowl.
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Grier is 19th and Gutekunst is 53rd (not shown above).
I included references to Gutekunst because Sullivan was working with him and it's the closest picture we can get to how Sullivan might have drafted, although very imperfect in that regard because we don't know how many, or which, Gutekunst picks he might have disagreed with.