The downside is that some seeders and leechers may not be able to connect to you. You have a high download speed because you were lucky enough to find a high speed uploader able to connect to you.
Newer versions of uTorrent support the uTP protocol (since 1.8.3), which is essentially a UDP firewall punchthrough. It is designed to improve speeds for people that can't open a listening port.
http://www.dslreports.com/forum/r22141654-Evading-throttling-with-uTP-uTorrent-19a
Open the port anyway otherwise all seeders you download from must support uTP which may not be the case in all torrents and your speeds will suffer.