IN the latest sign of continued activity in Hongkong's booming office sector, Emperor Holdings has bought the 17th and 18th floors of Nine Queen's Road Central for $234.4 million. A 20 per cent cash deposit has been paid. The floors have a combined area of about 26,000 square feet and the price is equivalent to about $8,550 per sq ft. The current tenancy agreement will expire in March 1996. Emperor property department manager Tony Tse said the acquisition was for investment purposes. ''Whilst the initial return is only four per cent, it will be substantially improved upon reversion in March 1996,'' he said. The Innsbruck consortium bought the building from Hongkong Land a year ago for $3.8 billion. At $8,200 per sq ft, it was considered expensive, but the market has since shown it can bear higher prices. Innsbruck recently sold another four floors of the building for about $500 million.