Like you, I bought my BTR5 a while ago. In fact, the price was about £80, then after Zeo review the price went up. Thankfully, I bought before his review.
The BTR5 has been a regular companion ever since. For me the quality I get from Bluetooth gives me the freedom to move around while I listen to review units and for my own pleasure. Is it worth upgrading to the BTR7? To me, the answer is yes, but do not get rid of your BTR5, as much as the BTR7 has improved the sound quality, better screen, bigger battery etc. the BTR5 is not shamed by its sibling.
On to your specific question, the sub-bass on the BTR7 and how it compares to the BTR5. This came as a bit of a surprise to me. To my ears, the BTR5 sub-bass is marginally more impactful. On the BTR7 the sub-bass seems a bit softer, although the bass quality is the same to me. I stress, the difference is marginal and could be a volume or tuning issue. However I set the volume at the same level both on my source (140 out of 150) and on both devices (15 and then 20). I used the KZ AS16 Pros (all BA) and IMR EDP Ace (DD, Planar and Bone conduction motor). The one variation in my test is that I attached a 4.4 to 2.5 adapter to the 4.4mm cables when using the BTR5.
All in all, the BTR7 is noticeably an improvement on the BTR5 to my ears and I am pleased I bought it.
I hope it helps you.