You can change the buffer size (from 256 to 8192 frames in ScripProcessor mode, it will be fixed at 128 frames in AudioWorklet mode).
Assuming your DSP code is polyphonic ready, you can activate the polyphonic mode, adjust the number of available voices, and test it with a MIDI device or application (usable with Chrome which implements the Web MIDI API).
ScriptProcessor: audio rendering is done using the old ScriptProcessor model
AudioWorklet: audio rendering is done using the new AudioWorklet model. This can only be tested with Chrome Canary for now, be sure to activate it