Ya clearly you don't know what you're talking about. Step up interest means the second half of your SSB purchases won't get the full interest.
Seeing as how the May 2017 issue gives me an interest of $593 on a $2500/year purchase, the CAGR works out to be ~2.152%
Extrapolating that information, the first 10 years no problem you can get 2.15% interest. What happens to the next 10 years where you need to roll the first 10 years of money but the interest isn't so high? This is the second half where compounding effect should matter the most because I have a bigger capital, but I do not have the full 10 years to receive the same step up interest of 2.15%.
All in all doing it this way will net you a 1+% CAGR.
Also I already gave SSB a better fighting chance here, by allowing myself a full $2.5k/year as a lump sum for this purpose.
So now, my question I've always been asking you is this. With the knowledge of SSB giving me 1+% CAGR on a regular lump sum of $2500 a year over the course of 20 years, I find that too little. I can sacrifice the 1+% interest for potentially higher returns, how do I execute trades that allow me to maintain my capital guarantee but give me potential higher returns of lets say 3%?
Edit: Take your time to figure out the answer, It should be difficult because I can't think how you could pull that off with SSB and ETF.
Edit 2: No excel, did 'paper napkin' math
I decided to help you out as well as show you the likely scenario.
Year 11 onwards I've already 'reinvested' the SSBs that matured along with their interest.
Interest earned column is total interest earned for the investment done in that year.