What's the maximum wattage suitable for a plastic cobrahead refractor? I don't want melting plastic.

Depends on the refractor, some can take a 400w HPS lamp, but usually 175 watt MH.

250w hps max is what i've seen

Plastic does not only melt, it also brittles. At temperatures which are high but not yet melty, it will crack after some time (usually years) in service. Heat also accelerates any UV degradation processes

The exact wattage depend on many things - including the size and material of luminaire (how much the upper part of the cobrahead is acting as heatsink), type of Plastic used, pattern on it, distance from the lamp and other things

I'm thinking of a 250 watt MH/MV in an AEL 115 cobrahead.

250 is a bit much. According to manufacturers specification sheets, 150 watt HPS, or 175 watt MV/MH is max for small cobraheads. My 150 watt HPS M250R2's acrylic refractor gets pretty hot.