How to mitigate BufferBloat on an ADSL connection?

It is probably bufferbloat. Tomato has no fixes for that.

Try openwrt, dd-wrt, or cerowrt's qos system, they all use fq_codel now.

I note that some forms of DSL use ATM encapsulation, accurate compensation for which was broken until very recently (fixed in linux 3.10.12 or later, current openwrt barrier breaker head, cerowrt head)

The way to fix "buffer bloat" is to disable your ISPs buffer entirely by limiting your incoming and outgoing bandwidth to just under their respective limits. This way the buffer on your ISP's side never gets used and you experience an improvement in latency at a small cost in throughput.

tc is the Linux/DD-WRT/Tomato tool for this. A script exists called "Wondershaper" that implements this on PCs, perhaps you could adapt it for Tomato.