* Mobis investors say as yet unconvinced of plan's value
* Glovis set for biggest daily percentage drop since 2015
* Plan will be put to shareholders for approval on May 29 (Updates shares, adds profit margin data, Samsung merger background)
SEOUL, March 30 (Reuters) - Shares of Hyundai Mobis dropped almost 7 percent on Friday, hurt by worries that a proposed restructuring plan would benefit the parent group's controlling family at the cost of the company's shareholders.
As a part of auto-to-steel giant Hyundai Motor Group's move to streamline its complex ownership structure, key affiliate Hyundai Mobis is to spin off its domestic module and after-service parts businesses and merge them with logistics firm Hyundai Glovis.
The plan will be put to shareholders for approval on May 29, but worries Mobis could be giving away cheaply what is seen as the more profitable part of its business will pose a challenge.
Glovis is personally backed by the auto-to-steel group's controlling Chung family, with Chairman Chung Mong-koo holding 6.7 percent and his son, Vice-Chairman Chung Eui-sun, holding 23.3 percent. The chairman has about a 7 percent stake in Mobis.
Uncertainty over shareholder approval drove down Glovis shares almost 10 percent, on track for their largest daily drop since 2015. The wider market was up 0.8 percent.
"From the perspective of Mobis shareholders, they are highly likely to vote against the deal, which is disadvantageous to Mobis and beneficial to Glovis," said Lee Jae-il, analyst at Eugene Investment & Securities.
"Should the deal collapse, Glovis shares, which rose on expectations of the merger, may lose ground."
Mobis investors have already said they were not convinced of the deal's benefits to the auto parts maker.
Deals that change the ownership structure of South Korean family-run conglomerates, or chaebol, typically attract closer scrutiny by institutional investors since the high-profile union of two Samsung firms in 2015 ran into shareholder opposition.
A merger planned between two Samsung affiliates in 2015 was opposed by U.S.-based hedge fund Elliott that said the deal "significantly undervalues" one of the firms and was not in the interests of the shareholders. The merger was finally approved by shareholders, resulting in Samsung C&T.
According to Mobis' presentation to investors, the business earmarked to be spun off had a profit margin of 10.3 percent in 2017, while the rest of Mobis had a margin of 4.7 percent.
As part of the restructuring plan, Hyundai Motor Group's chairman and vice chairman will sell their shares in Glovis to buy stakes in Mobis. Affiliates Kia Motors, Glovis and Hyundai Steel will sell their stakes in Mobis.
A fund manager noted there were signs in the deal structure that show the management took pains to take into consideration the Samsung merger, but he added it was not enough to convince him as an investor.
"This is a key business that is being transferred to Glovis ... it's a big hit to the business model itself," the fund manager said, declining to be named as he was not allowed to speak to media.
"We expect to vote against the spinoff in order to reserve our right to ask the company to buy out our shares." (Reporting by Hyunjoo Jin and Joyce Lee; Additional reporting by Dahee Kim; Editing by Himani Sarkar)