CARGO throughput at Kai Tak Airport is expected to drop by up to 10 per cent next year as a result of China's efforts to cool down its overheating economy, according to Hong Kong Air Cargo Terminals Limited (HACTL). HACTL managing director Anthony Charter said mainland efforts to rein in the economy would have an effect on Kai Tak throughput in 1994. ''I don't think China is going to slow down on its export drive. In fact, I think it is probably going to work harderon exports, if it can.'' But Mr Charter said there had been a growth in imports into China - some of which had been going to the mainland by air via Hong Kong. ''But, if China does clamp down on imports to try to cool consumer demand, it will have an effect on us. So I would expect our company to be budgeting for a slightly lower five to eight per cent growth in tonnage next year.'' HACTL saw a 12 per cent growth last year to 916,000 tonnes and has forecast a 12 to 15 per cent growth this year. ''It would be realistic to expect a slight slowing down next year,'' Mr Charter said. HACTL is already on target to break the million-tonne barrier in throughput this year. Although, the company has not increased its forecast, it expects the final throughput figure for 1993 to be about 1.05 million tonnes. Mr Charter also revealed HACTL was gearing up to join other bidders in making final submissions to the Provisional Airport Authority in January for the contract to operate the cargo terminal facilities at Chek Lap Kok. He said HACTL hoped to hear from the PAA on the successful bidder within a couple of months after that because the company needed to be able to place contracts in April, if there was any hope of hitting the June 1997 start for Chek Lap Kok. If HACTL wins the terminal contract at Chek Lap Kok, it expects to invest billions to provide bigger and totally new facilities. Nothing from Kai Tak will be carted to the new airport.