Some quick tables here using the incredibly handy "Splits" comparison available in the Play Index at Forman et fils. From the "stat extremist" point-of-view (and, admit it, if you're reading this you know you fit that description to a "T"...) the Reds continue to amaze in their case of gopheritis--and the breakouts here, isolating performance by starters and relievers--show just how pathetic things have gotten for the Cincinnati starters this year.
After being the worst in the league in '16, they've gone into the business of serving up gopher balls at such an enthusiastic rate (2.187 per 9 IP) that they are nearly half a homer per nine innings ahead of the next worst team in baseball history in terms of this particular split. (And the next three teams on that list--not shown here--are all from 2017: the Orioles, the Mariners, and the White Sox. The 2016 Reds are the "reigning champions" in this stat, but their 2017 brethren are a lock to own this record, quite possibly for the foreseeable future.
The data here can show us that the increase in HR/9 rate in MLB between 2016 and 2017 is pretty much evenly divided between starters and relievers. The individual changes, however, are much more all-over-the-map in nature. We can see that the Arizona Diamonbacks have done an incredible job of bucking the HR surge, with solid improvements from both pitching groups.
Odd to see that as the Reds' starters have jumped to a world-record pace in surrending HRs, their relievers have managed to go the other way--at least for now.
Of the three 2017 teams we mentioned who are outpacing the 2016 Reds starters in HR/9--the Orioles, Mariners, and White Sox--it's the Orioles who've cratered the most from last year.
And you will notice that the Cubs starters have also had a hefty uptick in HR/9 thus far in 2017 as opposed to 2016, when virtually everything broke right for them.
And we would be remiss not to mention the most incredible and calamitous performance change of all--the near doubling of HR/9 rate of the Washington Nationals' relievers. Their current rate is just ahead of the record set last year by the Reds' relievers--a pace that may not be possible to attain (and the N's have made some roster moves in the past week that are certainly designed to address the problem). Whatever they do over the rest of the season, however, it seems likely that they'll set a record for the highest HR/9 rate for any playoff-bound team. The current record? 1.329, held by the 2011 Texas Rangers (who came within one game of winning the World Series that year).
[EDIT: It should be noted that the current HR/9 rates for each pitcher group in 2017 is, currently at least, the highest such rate in baseball history.]