The 1997 Mariners were at only 145 through 91 games, and had a pretty slow stretch coming up. July was a very slow month of homers for that team (1.4/game) but they heated up in August/September (1.9/game). The season overall was 1.63/game (the Yankees are currently at 1.69/game).154 through 91 games breaks the SEA 1999 record for most HRs before the ASB (151) and puts NY on pace for 274 for the season.
Edit: And the 1999 Mariners were at 155 through 91 games, to add some context to Jon's comment. They were only at 87 games for the ASB.
Edit2: Not a comprehensive list of teams with more HR in first 91 games:
- 2000 STL 157 (Finished with 235)
- 2005 TEX 156 (Finished with 260)
- 1999 SEA 155 (Finished with 244)