Gear Questions

Are you sure about all this information? How do you know your cadence?  By the smallest Cog you mean you are in the one on the far right? Do you know your crank length?

On standard cranks spinning 80rpm in your biggest gear you should be a great deal faster than 20mph.

Cheapest and easiest would be to swap cassettes, but you're really only going to be able to go down to an 11 smallest cog. Switching to a standard crank is much more expensive (relative to a cassette swap)

However, if the gearing on your roadie is 50/34, you should not notice any difference on a tri bike with the same size compact crankset

You might want to play around with a gear calculator, like Sheldon Brown's.

http://sheldonbrown.com/gears/

Using some assumptions in a 50x11 combo (your road bike) you should be going 28.4 mph at 80 rpm.  Swithching to a 50x12 (your P2) would drop you to 26.1.  Based on those numbers it seems you would have plenty of gear on your P2.........

Sounds like you have a compact crank with a 50-34.

This will slightly lower the top speed gear by about 6%. (This article has a good explanation of compact vs standard cranks.)

Still, you should be way over 20mph if you're maxxing out a compact crank. Sounds like you don't have your rpm's high enough - I'd get those up to 90 minimum before you start talking about swapping out your crankset for bigger gears. 

Checking on the terminology here, seem to understand chainring size, but on the cassette (the cogs), by "smallest gear" do you mean the lowest? That would be the biggest cog and closest to the wheel. The "last" being ridden at your "fastest" would be the smallest cog or farthest away from the wheel.

I think you really want to know how fast you're spinning there, and also want to know what kind of situations are putting you into the fastest few gears. If you need to sprint and/or need a hill to use them, then it's likely not an issue. Being able to spin faster for peaks such as this would be more beneficial. 100 is a quick average, but it's really not that fast for such peaks. Since most of your riding is being done at the opposite end of the cassette, this would seem to be the case, but think you should check to be sure.

I live a few hours south of you and have been along the area between us before. Wisconsin by Lake Michigan is rather flat, but if you ever venture outside of the immediate area it can become quite hilly. Not necessarily Colorado length grades, but you can still see a couple miles in places and have some very steep grades at times. Both farther into Wisconsin and in the UP of Michigan. So check on the above things first, be sure of what's going on before starting to make changes.

Ok...assuming you are actually running out of gears at some point, there are some things you can do.

Change to a cassette that has an 11 tooth cog.  That will get you about 9% more top end speed which is quite a big jump.  At 85 rpms (which I would hardly consider spinning out), you could hit 30.6 mph.  At 105 rpms (which could be considered spinning out), you could hit 37.8 mph.  If you live in a flat area, it would take a serious effort and a big tailwind for most people to hit that speed on the flats.  And during training, rarely is it even necessary to go that fast.  Cost of a new cassette is generally around $60-$80

You could also swap out your chain rings.  But given you already have a 50/34, you can't only change the big chain ring, you would have to change both.  The 16t gap between the big and small chain ring is the most you want to go to have reliable shifting.  So you could go to a 52t big ring, but you would also need to go to at least a 36t small ring.  The difference between 50/12 and 52/12 is about 4% and at 85 rpms, you could hit 29.2 mph.  At 105 rpms, you could hit 36 mph. Cost for both chain rings would likely be around $90-$110.

Of course if you combined both solutions, you could end up with a 52/11 top end gear, which would get you 31.8 mph at 85 rpms and 39.3 mph at 105 rpms.  Total cost would be around $150-200

Lastly, you could swap your whole crank and go to standard 53/39 chain rings.  Cost would be around $150-250.  A 53/12 would get you up to 36.7 mph and 53/11 would get you 40.1 mph if you also swapped the cassette as well.

All that being said...for training, and especially triathlon racing, rarely do AGers (unless you're FOP) need to be concerned with top end gearing.  50/12 is plenty of gear for those purposes as you reach a point where tucking and coasting likely is more beneficial.  Now if you were a bike racer, and needed to sprint to the finish or bridge a gap on a downhill...then yes...top end gearing becomes a little more important.