Hi
This looks like an "MTU blackhole" problem to me. It happens when transit network is SILENTLY dropping packets starting from size X. Normally this should never happen: packets that are too large must be either fragmented or dropped with ICMP sent back to source.
I suggest that you try to use ping with different packet sizes (with dont fragment option) to figure out if this is happening. Then - try to understand where this is happening. TCP-MSS setup is good for increasing VPN performance, but it is not meant to workaround MTU blackholes. In particular, it only works for TCP traffic. Aslo, to have performance improvement, you need to adjust TCP-MSS on both ends.