The problem you face is that the GVP G-Force uses a proprietary 64pin SIMM. So, adding that extra 16M isn't going to be cheap. For example, Software Hut lists the 16M SIMM at ~$80!
But, I'd say this is your best bet. Kickstart ROM(s) aren't expensive, and are a worthwhile upgrade. (I couldn't stand the bootstrap, having killed my kickstart file more than once) I'd get this first and try the accelerator before buying additional RAM due to the cost though.
