Looks like it does eliminate boxing:

fun a(x:Float) {}
inline fun a(y:Number) = a(y.toFloat())
fun use() {
    a(1)
}

bytecode for

use

it straightforward:

ICONST_1
    ISTORE 0
    ILOAD 0
    I2F
    INVOKESTATIC org/jetbrains/ktor/application/ApplicationKt.a (F)V