Your textwriter is probably bottlenecked by the IO speed of the disk, it's easy for a mulitocre proc to greatly outperform a HDD. Having multiple writers is not going to remedy that, it's probably going to make it worse because of disk contention.
Assuming you have implemented an efficient wiriter, best solution is to have a maximum number of items in your queue and just accept the IO is the bottleneck here.
If you need to go faster, get faster disk (Solid State or RAID etc), or you could look at binary files and write things to them using a BinaryWriter. Or when you really need the very last drop of speed you can use structs and marshall them. Although this is risky if you want to move your files to other platforms.
Edit: forgot about protobuf.net, that would be my preferred solution to write and read data to disk. It's binary, fast and very easy to use.
Regards Gert-Jan