What is Structured Linguistic Literacy or Speech to Print? Is it based in the science of reading?

 Disclaimer: I've taught for 12 years and only heard the term speech-to-print within the last year. Since that point I've taken a deep dive and am sharing what I understand it to mean. If you're an expert in this field, please add to the conversation in the comments. I will edit the information based on you as a source or sources you provide. Thank you! 

TLDR: The official definition is this: "an accelerated, systematic, explicit and integrated instructional speech-first approach to reading and spelling." (see Shari Most, Project: Our Town)

Based on what I've read and seen I would say structured linguistic literacy (SLL) or speech-to-print methods are based on strong research. 

Background:

• Learning to read and learning to speak happen very differently. Researcher Philip P. Gough describes reading as "an unnatural act." Learning to speak in a native language requires no direct instruction, reading absolutely requires explicit instruction, of varying amounts, depending on the child. 

Speech to Print Implication:

• While learning to read doesn't happen by exposure, starting with the part of reading that is natural: the speaking serves children by providing a foundation they all have on which to build.
• This is why speech-to-print folks may disagree with the notion that reading is entirely unnatural. 
• Mark Seidenberg regarding Gough's statement about reading as an unnatural act (slides 18 and 19)

Background: 

• Knowledge Check: Which are there more of: sounds in the English language or spellings to represent those sounds?
• Answer: Way more spellings to represent those sounds. We have approximately 43 sounds in English, 18 vowels sounds and 25 consonants. In comparison, there are around 200 spellings to represent those sounds.

Speech to Print Implication:

• Beginning with sounds and teaching/learning their various spellings is way less demanding (on teachers and students) than trying to teach/learn every spelling and how it connects back to speech.
• Lessons progress from the simplest scenarios (sounds that are reliably represented by 1 letter) to more complex configurations: (1 sound can be represented with multiple spellings as well as 1 spelling that represents multiple sounds)

A note about the science: 

Is it based in science/research? Most speech-to-print folks will make reference to the work of Diane McGuinness (a researcher) and her book Why Our Children Can't Read and What We Can Do About It. I've heard yet to hear complaints about Diane McGuinness' scholarship so I'm assuming her research is well-respected.  Few people are studying/have studied SLL directly but the method was born out of research. Preliminary results are promising and we need more science! This quote from Dr. Stephen Truch feels appropriate: "Truth in science is never static. It's "whack-a-mole-ish. Let's be humble. We don't have all the answers and we only have an inkling of all the magnificence and complexity of reading."