Who reviews SBIR proposals?

Generally, SBIR proposals are reviewed by a committee at the federal agency where the solicitation came from. In short - they are not read by people from the Small Business Administration (SBA) which oversees the SBIR program. 

The committee typically includes the following types of reviewers:

• Sponsor point of contact (who works in a lab inside the federal agency that released teh solicitation)
• Academics
• Government lab researchers 
• Corporate researchers in industry
• Other technically competent people in the community

Given several of the reviewers are likely not working directly in your field, you will need to work hard to make the first 3 pages of your proposal interesting and comprehensible while not making it condescending. The subsequent sections should be technical but draw from context provided in the first 3 pages. It's a fine line and it is always worth investing in making the proposal human-readable. Budget at least 2 weeks to write the proposal so you can test it out with other people and iterate.