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O.K., it's of course necessary to account for the source impedance when calculating the resistor values. I presume you know that it's essential for achieving a high current source output impedance. But it doesn't change circuit bandwidth.
A your previous bandwidth measurement was apparently affected by load capacitance (and possibly other parasitic circuit capacitances), we are still waiting for a correct measurement of the actual circuit bandwidth.
But apart from issues of measurement setup, there are limitations of feedback active current sources.
From a practical point of view, the current howland current source is only as good as CMRR of the circuit, which in turn depends on the tight matching of the resistors. Here is a report by from TI.
If you still wanna go down this route there are differential amplifier packages with laser trimmed resistors for precise matching which would be better suited for this purpose.
Alternately, you can build an effective current source with BJT's at that frequency at a much lower cost if you follow circuit in post #7 in the following link:
I say this because I have learned the hard-way that; although simulations are essential for any circuit design; implementing in practice is a whole different world, as experts here would agree.
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