Conclusions section in a letter: To give a peek into new findings (not reported in letter) or not
It sounds like the depth of your findings are at odds with the letter format. If you want to talk about the full optimization across all parameters in detail I would consider an alternate format for publication.

That being said, one of the prominent journals in my field (JASA, whose full author guidelines can be found here) provides specific guidance for letters to the editor that supports your mentioning additional results that will be expanded on in future publications:


  The work should have a modicum of completeness, to the extent that the letter “tells a story” that is at least plausible to the reader, and it should have some nontrivial support for what is being related. Not all the loose strings need be tied together. Often there is an implicit promise that the publication of the letter will be followed up by a regular research article that fills in
  the gaps and that does all the things that a regular research article should do.


Note that this is just one journal in one field, and the expectations of the journal you are submitting to may differ.  I would recommend looking at your target journal's guidelines before proceeding one way or the other.