eSATA, USB3, Firewire800... whatever!.. is it worth it?

I recently purchased an external eSATA/USB2 enclosure for a drive(2Tb), and have only just started using it in anger.

Speeds are not exactly how I imagined them to be. No problem with read speeds tho.
my first experience with it was good @ 40-45MB/s.

My first experience with writing to it(in anger!).. 250(odd)Gb of files off an external USB2 drive(@1Tb) wasn't too bad. Averaging about 20MB/s.
I'm using Win7's copy and paste displayed transfer speeds as a guide. because there was about 3-5hours of transferring time, I came in and out and did other stuff on the PC concurrently.. just having a quick squizz to see how fast things were travelling along.
My transfer times are not in any way a scientific analysis, just quick observations at any given moment.

Today I transferred another 50G of data from an internal SATA drive, to the eSATA drive again.
Speed started off nicely at 45MB/s for a few minutes(maybe the buffer was doing it's job??), then slowly started to drop, steadily, but worryingly too quickly.
It got to as slow as 16.9MB/s for a long time. Thinking that this was some large group of small files or something, I sat and watched it for a minute or two to see how long it'd last at that speed... I gave up before two mins!(boring! ).
16.9MB/s is not what I expected at all.

I was hoping for at least 2x USB2 speeds, which I regularly get to 10-12MB/s over a long transfer time.

I realise that transferring via Windows's copy/paste method may not be ideal, and I do have a backup program called RichCopy, which allows multiple threading and stuff like that(which I don't fully understand, but kind'a do!) and using that I can get approx 12-15Mb/s write speeds to one of my 1Tb USB2 drives, which is about 50% full.

I also did a read test, from the eSATA drive back to the internal SATA drive, and got a min 50MB/s. So the drive is fast, and the eSATA interface is also fast enough in read mode.
But!... at a guess, it seems that a realistic write speed would be more like 20MB/s, which is what I have regularly seen(except for that slow '16.9MB/s' stage just a few minutes ago).
I was kind'a hoping to see write speeds more like 1/2 read speeds(ie. at least 25MB/s) and hopefully 30MB/s.
I do get these speeds for small transfers, like a few hundred Mb's or so, but once the amount of data starts to get to 500M or 1G, then thing slow down dramatically.

I think Win7 has a performance indicator tool somewhere that I once saw.. don't how to access it, or if it measures hdd performance. I'll get HDTACH again to do more accurate testing again.

Anyone else have USB/eSATA/FW drives and noticed the performances of the drives?

I've seen USB3 external drives offered for sale. I'm loathe to purchase one, just to find out that the speed advantage is minimal. My motherboard does have USB3 capability too tho.

Are there any tips or tricks, apart from the larger block size, that can help to speed up a hdd?