Tuning TCP: Should I not be using SDL_Net?

I'm building the networking for an MMO engine and am working on tuning the TCP performance. In wireshark, I've been seeing slowdown scenarios like this:

T=0ms: Client sends 29 packets with len=500
(sends are accumulating)
T=54ms: Client receives acks for all 29
T=54ms+: Client starts sending again with len=1460

There's 2 things going on here:

1. It seems like the send buffer is getting full (I can send just about 16kb before it starts waiting).
2. Messages are being packed into max-MTU packets if there's multiple messages available (I static-compiled SDL_Net with TCP_NoDelay …