Based on the records below:
Weeks of training data by trainer level:
1: 159
2: 191
3: 241
4: 7906
5: 6763
6: 3781
7: 380
http://training.bb-usa.net/club/showTrLevEffect.php
The chart seems to indicate that level 5 trainers offer little value over level 4 trainers.
Personally a level 4 trainer with career extension for $13,000 a week seems much more valuable than a level 5 trainer costing $22,000. Saves me 9,000 a week and gives me career extension for nearly identical training speeds.