Exactly - if I understand you right you are saying that like elements at the semantic stratum can be combined and this can result in dissimilar elements being coordinated at the LG stratum if these dissimilar elements realise the 'same' semantic function. So, grammatically we have different ranks being coordinated but for good semantic reasons - just another example of interstratal tension. That would be exactly my opening gambit.
 Bartlett's proposal is that SFL theory would be improved if paratactic extension is held to obtain between units of different ranks. Apart from the misunderstandings involved — identified here and here — such a proposal creates theoretical inconsistencies with regard to both constituency and complexing.
 'Interstratal tension' is Martin's characterisation of grammatical metaphor. None of the examples provided by Bartlett involves grammatical metaphor.