How to batch process many clips for adding pixel motion blur

Scripting can certainly apply effects/presets.    Not sure if I could help yo with the caching thing,  but does it really matter as AE will simply purge older data as it creates new data while it's rendering?
But here's one method you could use to get the job done.  
Bring your clips into AE, drag them onto the Create a New Composition icon on the bottom to create your 300 comps.
Create an Animation Preset of Pixel Motion Blur with your prefered settings,  then use this handy script and it will apply that preset to all the layers in all the comps.  ***BE sure to change the section in // --- UPDATE THIS TO YOUR REAL .ffx FILE PATH ---. to the location on your system where your presets are stored.

(function dockablePanel(thisObj) {

function applyPixelMotionBlur() {
app.beginUndoGroup("Apply Pixel Motion Blur");

var sel = app.project.selection;
if (!sel || sel.length === 0 || !(sel[0] instanceof CompItem)) {
alert("Please select one or more comps in the Project panel.");
return;
}

// --- UPDATE THIS TO YOUR REAL .ffx FILE PATH ---
var presetPath = "C:/Path/To/Your/Presets/Apply Pixel Motion Blur.ffx";
var presetFile = new File(presetPath);

if (!presetFile.exists) {
alert("Preset file not found:\n" + presetPath);
return;
}

for (var i = 0; i < sel.length; i++) {
if (sel[i] instanceof CompItem) {
var comp = sel[i];
if (comp.numLayers > 0) {
var topLayer = comp.layer(1);
try {
topLayer.applyPreset(presetFile);
} catch (e) {
alert("Failed to apply preset to: " + comp.name);
}
}
}
}

app.endUndoGroup();
}

function buildUI(thisObj) {
var win = (thisObj instanceof Panel) ? thisObj :
new Window("palette", "Pixel Motion Blur", undefined, { resizeable: true });

win.orientation = "column";
win.alignChildren = ["fill", "top"];
win.spacing = 10;
win.margins = 10;

// --- Simple text message ---
var msg = win.add("statictext", undefined, "Select Comps in Project Window");
msg.alignment = "center";

// --- Button ---
var btnApply = win.add("button", undefined, "Apply Pixel Motion Blur");
btnApply.onClick = applyPixelMotionBlur;

win.onResizing = win.onResize = function () { this.layout.resize(); };

return win;
}

var myPanel = buildUI(thisObj);
if (myPanel instanceof Window) {
myPanel.center();
myPanel.show();
} else {
myPanel.layout.layout(true);
}

})(this);