I might be off the mark, but from what I understand, setting the RTT lower will mean that Cake can clamp down on packets faster when you are hitting your bandwidth limit. It allows it to act faster as it is expecting the packet acks much sooner.
I also believe that you need to set the RTT to a value that is somewhere around the average ping to the Internet services you are accessing, not the first hop. The packet round trip is measured out to the destination, not your first hop, so in order to guesstimate when to expect replies for your traffic, the round total round trip must be accounted for.
Reading the docs seems to suggest that cake self tunes the RTT pretty well, so getting it in the balllpark is good enough. The default value of 100ms should be fine for most cases. You may consider "Regional" (30ms) if you are mostly accessing local game or streaming servers or something like that. I wouldn't go any lower unless you had very good reason to though.
Cake is about preventing a log jam on the interfaces of your router and the one it connects to. Keep it simple and only adjust the bandwidth parameters. When you lower them sufficiently, they prevent the interfaces from being a bottle neck. Increasing RTT will have the same effect as lowering bandwidht. By adjusting multiple parameters you are simply making it harder to understand the effects of your changes.
Cake can not fix upstream bottlenecks. The best fix for this is a good ISP if one is available.
Morris