Business is already unhappy with compromise civil rights legislation now rushing through Congress. But worse may lie ahead. Large employers, fearing a wave of lawsuits, feel that the proposed $300,000 cap on damages for victims of sexual discrimination is too high. But only a last-minute deal in the Senate prevented a move to eliminate the caps entirely. Senator Tim Wirth (D-Colo.) agreed to hold off on an amendment supported by women's groups. In exchange, Senate Majority Leader George J. Mitchell promised a separate vote next year on legislation lifting the ceiling.