Well, we can check out his Fielding Runs Above Replacement (FRAR) and Fielding Runs Above Average (FRAA) adjusted for all-time to see where he was at.
To make things a little easier, let's look at his 5 best consecutive defensive seasons, or defensive peak I guess you could call it:
There are a few really good years thrown in there, along with a 1990 season in Texas where he was 10 FRAA and 27 FRAR. Overall for his career:
Thats 21 FRAR a year, and roughly 7 FRAA per year if you want to average it out.
I hate to do this to you since your an Angels fan, but I'm going to select Jim Edmonds as the comparison. I don't think Edmonds is the greatest centerfielder defensively of all-time, but I figure I'll take names off the top of my head until Pettis loses out to one of them.
Jim Edmonds Defensive Peak
2005 (so far)
Now it looks like Pettis beat out Edmonds due to the extra FRAR and FRAA, but that is one of two peaks Edmonds has. Outside of Pettis' one peak, and that additional 10 FRAA 1990 season, there are no real impressive defensive performances for him. 59 of his career 67 FRAA are tied up in 6 of 10 seasons. Now 3 of those seasons are very impressive, with 17, 16, and 10 FRAA, but that is not enough for me to say he is the best defensive centerfielder of all-time. I'll see if I can hit upon a definite who for centerfield at some point, but for now we know who it isn't.
Age 24 1975: 2.1 WARP1
Age 25 1976: 5.9 WARP1
Age 26 1977: 9.3 WARP1
Age 27 1978: 5.6 WARP1
Those are Bostock's four years of playing time in the major leagues. The 9.3 season looks like it could be an outlier (we'll obviously never know) but considering it is flanked by a 5.9 WARP and a 5.6 WARP, it is safe to say that he power spike he had (14 of his 23 career homeruns were hit in 1977) was not his real power. His doubles totals were also up (36 doubles; 21, 21, and 24 for the other three seasons). 12 triples in 1977, but 9 the previous year, and 5 and 4 in his other two seasons, so that atleast looks like a repeatable stat. Triples are sort of fluky sometimes anyways, and it is considered a speed thing more than power. The point of me picking apart differences between 1977 and the rest is because you could imagine him to be the 9+ win all-star player from 1977, or you could imagine him to be what he would have been; a 5-6 win important piece of the puzzle for a few more years. He was only 27 in his last season, so it is safe to say he would have most likely followed with 4-5 more years of 5-6 WARP per season. Not star quality, but its the middle guys that teams can't seem to get enough of, so there is importance in what he would have become.
My favorite kind of question, joy!
Here's your numbers for the average Hall of Fame right fielder...
Career WARP3: 110.2
Peak WARP: 43.3
...and Cooperstown left fielders:
Career WARP3: 103.8
Peak WARP: 42.8
Salmon is the right fielder, and Downing is the left fielder. Let's check out their respective career figures:
Tim Salmon 1992-2004
Career WARP3: 77.4
Peak WARP: 42.3
Salmon's main problem is injury; notice his Peak WARP is only 1 point below the average Cooperstown right fielder. His career WARP3 is much too far away though, so he can be thrown into the list of "guys who shoulda coulda woulda had they been healthy". He had the talent for the Hall, but not the body sadly.
Brian Downing 1973-1992
Career WARP3: 93.6
Peak WARP: 31.9
Alot of Downings' value was tied up in the fact that he played for 19 seasons, rather than being dominant at any one point. He has one 9.2 WARP3 season, and another 7.4, but besides that it is all 6's, 5's, and 4's. There is value in that, as mentioned by Bostock's career, but Downing was not a Hall of Fame caliber outfielder. Tim Salmon is, but injuries kept him from it. Luckily, there is a place for both of them in the Ray Lankford Wing of the Hall of Fame.