Your upload bandwidth is more important than your download bandwidth. A 40/40 Ren-X server uses about 3 mbit/sec upload, 750 kbit/sec download.
If you have sufficient bandwidth, that's likely not the issue. Line quality is determined by jitter (variation in latency) and packet loss. If clients have high jitter and packet loss, the game will "lag" for them even if their average latency is low.
In addition to line quality, the CPU of the server plays a big role. Ren-X demands strong single-threaded performance. Which CPU are hosting this server on?