You could conceivably achieve a 2.75\" gain with PMMA, or a combination of PMMA & the silicone implant, but I strongly suspect the chances of very poor aesthetics are very high. Such a change would dramatically dwarf the glans and the penis would come off as being very obviously enhanced. On top of that, a Girth of 8\" may actually make some sex (especially oral & anal) prohibitive, and the PMMA isn\'t easily reversible. There is already much discussed potential for complication, especially those detailed about the implant on this forum. PMMA\'s long term implications are not well known, making such a massive gain all the more risky if things go wrong down the road.
So in a nutshell, a 2.75\" is conceivable, but very inadvisable! I\'m not sure if you were talking in hypotheticals with the numbers you stated, but if you are indeed 5.25\", you are arguably thicker than average as it is.