The Jurassic period. O2 in atmosphere was 130% modern levels. CO2 was at 1950ppm, 5-7 times modern levels. The temperature was a whole 3 DEGREES C over modern times!
The Jurassic period was really quite long, and long ago. Long enough for solar evolution to be significant. At the beginning of the Jurassic, the sun was about 2% fainter than now, at the end about 1.5% fainter. That is about 26W/sqm on the solar constant, or about 4.6 W/sqm of radiative forcing if corrected for albedo and averaged over the whole surface of the Earth. 5 times modern CO2 is about a radiative forcing of ln(5)*5.35, or 8.6W/sqm. So just the change in the sun cuts the effect into half, leaving 4W/sqm, which our current climate models translate into 3.2K of temperature difference. So even without taking other effects (minor orbital variations, configuration of the continents) into account, your claim agrees quite nicely with our current theoretical results. Of course, the sun is unlikely to get significantly fainter or stronger over the the next few thousand years, so there will be no free lunch from that angle. If we go back up to 5 times current CO2, we can expect about 7K of temperature increase.
And who wants more CO2 @1950 ppm, you know, to make all those plants and trees convert that CO2 into a higher O2!
Since our increase of CO2 produced by burning fossil carbon with atmospheric oxygen, at best we'll get back the O2 we sucked from the atmosphere. Not that a significant quick increase would be advantageous - it would play havoc with the biosphere and massively increase the risk of and by fires.