My friend told me I'd lose travel. Is this true?

The increased leverage moves the wheel further into the travel arc. There is NO travel loss. Your friend spends too much time on the forums reading so-called experts who didn't reset sag or get the proper spring rate for their fat girlfriend.

  Will I need a heavier spring with a YamaLink?

Stock WRF, YZ & YZF suspension (not the WR250R/WR250X) requires approximately 100mm of sag with rider weight limit of about 150 - 165 pounds BEFORE gear. With a YamaLink we recommend a 20% heavier spring if you weigh close to 160 pounds. This does NOT apply to any RACE edition YamaLink.

  How do I set sag & why do I have to reset sag?

YamaLink's extra leverage improves suspension by helping it get more traction, turn better & ride more plush, but if you don't reset sag and properly spring the bike it will bottom easily, turn slowly and ride like a Hodaka Dirt Squirt with a flat tire....and then all the self-professed suspension experts post on every forum anecdotal doomsday stories about their bad lowering link experience.