So, just to be clear, the issue is that the tape runs too fast relative to the emulation speed? If so, this explains some of the troubles I'm having with emulating the 300-baud tape interface as well. I have 99.9% of the bits coming into the machine just fine, but every so often, the emulated machine 'misses' a bit - presumably due to tight timing.
I'll see what I can do to mitigate this issue, but I'm afraid a proper solution may be out of my hands, as it could require a total re-write of the driver to have more accurate timing. If that invalidates the bounty payout, so be it, I understand.