RocksDB performs much better and is the most popular and efficient KV store in the market, being used at both Google (Leveldb) and Facebook. Therefore, the benchmarks are against that. Without spending time generating benchmarks, I'll bet that Badger would beat BoltDB any day.
The idea of using B+-trees with value log has potential. One would need to do some obvious optimizations to decrease the frequency of writes to disk. Because SSDs have to run garbage collection cycles, which can affect write performance in a long running task.
But, I think it would make a great research project. And if it comes out to be better than our current approach of LSM tree in read-write performance, I'd switch in a heartbeat; because I think B+-tree might be a simpler design.
BoltDB author here. I agree with you that Badger would beat Bolt in many performance benchmarks but it's an apples and oranges comparison. LSM tree key/value stores are write-optimized and generally lack transactional support. BoltDB is read-optimized and supports ACID transactions with serializable isolation. Transactions come with a cost but they're really important for most applications.
Regarding benchmarks, LSMs typically excel in random and sequential write performance and do OK with random read performance. LSMs tend to be terrible for sequential scans since levels have to be merged at query time. B+trees are usually terrible at random write performance but can do well with sequential writes that are batched. They tend to have good random read performance and awesome sequential scan performance.
It comes down to using the right tool for the job. If you don't need transactions, Bolt is probably overkill. There's whole section on the BoltDB README about when not to use Bolt:
> It is just badly implemented, acquires a global mutex lock across all reads and writes.
You're welcome to your opinion about it being "badly implemented" but the global mutex lock across reads and writes is simply untrue. Writes are serialized so those have a database-wide lock. However, read transactions only briefly take a lock when they start and again when they stop so they can obtain a snapshot of the root node. That gives the transaction a point-in-time snapshot of the entire database for the length of the transaction without blocking other read or write transactions.
So, you should really test your read/write ratio with the size of the keys and payloads before selecting one solution or another. Even on the Badger tests, you can see that it can vary a lot.