Martin (1992: 371-2):
Australia II is an Actor in [5:36kk] but a Car[r]ier in [5:36ll]; similarly the Americans are a Medium in [5:36y] but an Agent in [5:36dd]; and 'being ahead of' is variously realised as a Process [5:36kk], Attribute [5:36ll], Goal [5:36y] and Circumstance [5:36dd].
Boats leading kk Australia II + leading x by a couple of = boat-lengths ll Australia II + leading[was ahead] x by 21 seconds y Americans + leading[increased lead] x to 46 seconds,at second = mark dd Liberty + leading x more & more[surged ahead] Fig. 5.36. Nuclear structures for ‘leading’ in text [5:36]
 Clause [kk] — like clause [ll] — construes an attributive relational Process. Viewed 'from above', it construes 'being–&–having', not 'doing–&–happening'. Viewed 'from roundabout', the unmarked present tense for lead in such a clause is the simple present leads, rather than the present–in–present is leading, which rules out the material Process analysis.
by the same margin
she (Australia II)
21 seconds ahead
around the fifth mark
 In clause [y] the Americans serves as Agent, not Medium, and in clause [dd] Liberty serves as Medium, not Agent.
at the second mark
to 46 seconds
Location: motion: away from
 In clause [kk] 'being ahead' is construed as Process and circumstantial Attribute, not just Process, and in clause [ll] also as Process and circumstantial Attribute, not just Attribute.
 The relation between Medium and Process is complementarity, not extension, for reasons previously given.
 The relation between Agent and Nucleus is enhancement, not extension, because the Agent is the external cause of the Process unfolding through the Medium.