(MONEY Magazine) – Investors have a seemingly insatiable appetite for new stock offerings by hip full-service restaurant chains, notes money manager Michael Harris. Among them are such hot issues as $34 million Lone Star Steakhouse (up 465% since its market debut a year ago) and $122 million Outback Steakhouse (up 475% since mid-1991). Rationale: Sitdown dining for under $20 a person has growing appeal for two-career couples. Although such chains can be terrific investments (see ''Beating the Street,'' page 120), Harris says the days of rapid growth and fat profit margins are probably behind 23-unit Lone Star and 85-unit Outback. ''Investors, who've grown accustomed to 10% to 15% gains in same-outlet sales, overlook the fact that both firms are now operating near capacity,'' he says. ''The only way that kind of growth can continue is for prices to rise, which is unlikely in this dog-eat-dog field.'' So he's dumped his portfolio's 6% holding of Outback at about $25 (for a 150% gain in 15 months), and he wouldn't even nibble at Lone Star.