What range of tcp buffer size do you suggest for slow and fast networks?
Bufferbloat only becomes a problem when the link (i.e. your edge/gprs connection) is congested. If it's not congested, you may just have very poor cellular reception.

If bufferbloat is really the reason you will be unable to do anything locally (because the problem is very probably the largish buffer size on the cellular network equipment).

A few pointers:


try to avaid parallel traffic when using WhatsApp
try to switch off Image loading in WhatsApp (not sure if this is possible as I don't use WA)
(root required) use an outbound app firewall to avoid unnecessary traffic (e.g. AFWall+)
(root required): use an adblocker to avoid unnecessary traffic (e.g. AdAway)