The cheapest way? Use a µC with USB built in. Many of the pic18 and upwards have this. So do many other makes of µC.
The simplest way? As you already state - convert it to RS-232 at TTL levels with some form of conversion chip like the FTDI.
Yes, there could well be problems with drivers for both solutions, but it is more likely the FTDI will be supported out of the box. However, the first solution has the advantage that you can make the µC look like whatever device you want it to look like.
Oh, and as an aside - some of the high end PICs (eg some pic32 chips) have USB host built in as well. Just thought I'd mention it...
At some point in my life, I used to run the USB business for big semi company. The best result I remember was NEC SATA controller capable of pushing 320Mbps actual data throughput for mass storage, probably current sata drives are capable of this or slightly more. This was using BOT (some mass storage protocol runs on USB).
I can give a technical detailed answer but I guess you can deduce yourself. What you need to see is that, this is ecosystem play, any significant improvement would require somebody like Microsoft to change their stack, optimize etc, which is not going to happen. Interoperability is far more important than speed. Because existing stacks carefully cover the mistakes of slew of devices out there because when the USB2 spec come out probably the initial devices didn't really confirm to the spec that well since the spec was buggy, the certification system was buggy etc. etc.. If you build a home brew system using Linux or custom USB host drivers for MS and a fast device controller you can probably get close to the theoretical limits.
In terms of streaming, the ISO supposed to be very fast but controllers do not implement that very well, since 95% of the apps use Bulk transfer.
As a bonus insight, for example, if you go and build a hub IC today, if you follow the spec to the dot, you will practically sell zero chips. If you know all the bugs in the market and make sure your hub IC can tolerate to them, you can probably get in to the market. I am still amazed today, how well USB is working given number of bad software and chips out there.
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I've never used one, but I'd look into the AT90USB1287 chip. It even comes in an handy evaluation board called the AT90USBKey. While the chip itself is supposed to be able to act as a USB-OTG Host, it isn't clear to me that the demo board supports that.