Regardless allocation settings files always has Sparse attribute.

I guess you are using Windows. There are two ways to relieve this issue.

For small torrents
1. download them to RAM disk.
2. then seeding in HD/SSD.

For big torrents
1. set download them to HD/SSD and start.
2. stop downloading immediately after files has been allocated as sparse.
3. delete files and re-allocate them as fast allocate, with same size and timestamp.
4. go on downloading.

Step 3 (big torrents) needs another program, I wrote one by use python. I have posted a test case here before, but cannot find it now.

if you really want to pre-allocate (I haven't tested if it's broken on my instance fwiw) probably the easiest way to work around this on Windows is:

1. stop the torrent
2. find the file/folder in explorer, cut it and paste it on another drive letter.
3. Cut it back to the original location, start the torrent again.

you could even slightly automate this and have an incomplete folder on the other drive, the resulting move will remove  any fragmentation it would have from being sparsely written.

I assume you are trying to avoid fragmentation? if this is why you want to pre-allocate your torrents I highly suggest you use sequential downloading instead which will give you a nearly sequential file on a Win/NTFS filesystem, which you can defrag nearly instantly. (you can do this by right clicking files in a transfer or changing the global trigger for this)

No, we donot want any pre-write, just want the fast allocate.