I second the speculation that since more US citizens speak Spanish rather than Portuguese, the "mais" (shouldn't it be "maíz", btw? - that's what my Spanish dictionary says) sounded funny to and "mas" made more sense to someone proofreading texts along the way - "Mas Que Nada" makes sense in Spanish. (Even the Latin Real Book has it "Mas Que Nada".)
Got the 2 CD set today - it's a good selection. Saves me quite a lot of money on the individual CDs with their short playing time, and for the many less intricate pop tunes on them this is enough Mendes for me.
Also got the "Primal Roots" CD - I love it, great to hear it without scratches and tape hiss after all these years, and stick my earlier post that it is THE BEST RECORD Mendes has made in his entire career!