Current atmospheric O2 levels are about 21%, though the Oxygen compensation point dictated by C3 plants who produce our O2 limits it to about 23% at current CO2 levels. Higher CO2 levels permits increased photosynthesis rates and a correspondingly higher atmospheric O2% level.
220 ppm CO2 has a upper O2 limit of 23%O2. 350 ppm CO2 has a upper O2 limit of 27%O2. 700 ppm CO2 has a upper O2 limit of 35%O2.
These limits are "theoretical max" which you'll never reach due to O2 consumption from both organic metabolism, and from inorganic O2 fixation (I.e. rust and other metal oxidation). Once you hit the O2 compensation point, plants stop growing... they reach a point where the O2 levels provide a compensating force on enzymes that halt production of Rubisco.
Whats this mean in modern terms? well... we have increased volcanic activity, massive forest burning in Thialand and Brazil which has driven up CO2 levels since 1800. The increase CO2 raises the maximum O2 concentration and hense the plant-based biolevels possible (ie. more crops, faster growth of plants/food).
global O2 levels move VERY slowly, though its worth noting that past CO2 levels around 1500 ppm correlated to about 35%O2, so even at high compensation points (max levels) they never reached very high.
Humans can supposedly breath 50%O2 all day long without issues, and Scuba Divers like myself can get Nitrox-certifications to use 40% Nitrox (40% O2 with 60% N2) for shallow dives (the higher O2%, reduces the %N2 in the mix to slow nitrogen gas uptake in the blood... basically we use it to do longer dives without having to decompress).
Your question about 30% is interesting. 1) it would give endurance athletes a higher "effective" VO2 Max and would theoretically allow marathon runners to run at slightly higher speeds/effort while staying in the aerobic-exercise zone. This doesn't mean they could go farther, total energy is based on calories available... only that they could burn the same energy faster. 279
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2) Sprinters and other anaerobic activities would be unaffected... except for recovery rates! Hockey players, basketball, soccer, etc where stop and go windsprints are common could see reduced recovery times (i.e. hockey players could go back on the ice after shorter breaks without lactic acid building up). Even in todays pro sports you see this on football sidelines and hockey benches where winded all-stars will dawn a O2 mask to reduce lactic acid. So you could sprint more often, but not necessarily any faster.
3) Insects wouldn't be bigger. The giant insects in eras past were originally thought to be from O2 absorption, but studies in hyperbaric chambers have shown that O2 absorption rates are not limiting factors. Most scientists attribute ancient giant inserts to a lack of predators in that era, and an abundance of food. They simply lived long and ate well (in addition to theories about indeterminate growth and genetic differences in ancestors).
4) Fire hazards may be an issue, though an increased oxygenation rate of metals and spoiling of foods would be the most common issue for sure.
5) Lastly, its worth noting that CO2 levels in your blood are what trigger your brain to breath. The peripheral chemoreceptors in you carotid arteries will trigger breaths when CO2 concentration rises to 40mmHg. The CO2 levels in your blood vary between breaths from 35 to about 45mmHg are generated by metabolic means. So will increasing atmospheric CO2 levels affect your ability to breath? Nope. 40mmHg which is found in your blood and lungs alveolar space is 53000 ppm... which is why it diffuses out of your blood and into the lung space (since air is only ~400ppm). Increased CO2 and O2 levels shouldn't affect your ability to trigger breathing unconsciously. ;) 279