Those are great questions!
The block interval does not impact scaling in any way other than the fact that it is part of defining the maximum growth of the blockchain itself (not state size). max_block_size / block_interval is the maximum growth rate for the blockchain in bytes. We are not experiencing any problems in the live performance of the blockchain. We would likely only consider changing the block interval if there was a compelling reason to for the benefit of live performance.
Some of the changes we have been working on do specifically target response times for our APIs. Appbase parallelizes all API requests. Previously, requests were handled serially. This doesn't scale well and is a waste of resources as almost all computers now have multiple CPU cores. With Appbase, we are now utilizing the other 90% of our servers that weren't being touched previously.