Conclusions section in a letter: To give a peek into new findings (not reported in letter) or not Perhaps because this is my first letter, that I am writing independently, I don’t have a good grasp of the amount of significant results that need to be in the letter, to be published in the journal.

Because this is a letter, I cant just go all in to describe every possible result we have obtained, so I only showed a small portion of the results. 
While, in a certain light the results shown in the result are small, extrapolating it to a larger problem, the results are way more significant.
For example, in this letter, I present the first model for a certain device. I experimentally validate the model and show the variation of a certain parameter on the performance of the device. A 10% variation in the parameter has a 10% impact on the performance of the device. However, the 10% is a hard-limit, and you cant really optimize that parameter beyond ~10%. However, there are a multitude of other parameters which can be similarly optimized, and I have obtained theoretical results showing >50% and 1000% theoretical improvement in the performance of the device based on 2 performance metrics. 

However, going into details of these individual parameters and their optimizations is almost impossible in this short letter.

I know that conclusions are not supposed to have any results, but are supposed to discuss the implications of the results. Can I consider the broader results (50% and 1000% improvement in theoretical performance), as implications of the current study? Can I report this performance improvement in the conclusions section (without giving too much details of the study), as a study we have done and mention that we are going to report it in future publications?
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.