Richard - first thing to do before modifying the boat's design is to sail it! I would not do major surgey until you understand how it all works under sail. The cockpit drains on my contessa are 1.5" ID,(I think!), sounds like maybe yours are smaller? Inadequate or adequate is really decided by where and what conditions you sail in. For any coastal cruising, the cockpit drains are absolutely fine. For offshore cruising, the bigger the better.
The reason you'll never see cockpit drains out the transom smaller cruising yachts is because they will put more water into your cockpit then they'll remove - by a LOT. Think it through - you're talking getting 3 inches of drop. The only time you're going to get a cockpit full of water is in breaking seas, likely over 12 feet, and likely following seas. Picture your boat going down a 15 foot cresting wave. Where are those transom cockpit drains in relation to the cockpit now? They are a foot or more above your cockpit sole and under the pressure of many tons of seawater, and are flooding your cockpit rather than draining it. That would be a very big problem because while they would drain a bit after you crested the wave and the trim of the boat went the other way, (bow up), they would drain slower than they flooded as the outside pressure of the ocean would be greater than the pressure of the amount of water in the cockpit.
So assuming your concerned about the drains because you're going to be sailing offshore, I would suggest 1 of 2 things, or both:
1. Get an SSB and a subscription to a routing or weather service for sailors. The best way to avoid a swamped cockpit is to never put yourself in the conditions that would cause it.
2. If you really want more efficient drains, leave the existing routing right where it is, (crossing over is very important again not to flood your cockpit); get out some power tools and make the existing ones bigger! It should not be a big job and would probably make them quite adequate.
Ball valves or no - I think Merrill was right and wrong. He's right in that they would likely get frozen and are hard to reach so might be no help; on the other hand if you have them on the cockpit drains, they definitely won't hurt. My 26 does have ball valves, and one was frozen when I bought her. I've since freed it and if you grease them once a season, and just operate the levers a couple of times every time you have the engine hatch off they won't freeze again.
Another note on transom openings in following seas - you generally DO find the wet exhaust outlet there, but often find no way of closing it. Just as large following seas could flood your cockpit if you installed drains there, they could, (and have on many boats), also flood your cylinder head with seawater, so would definitely recommend installing a way to close the wet exhaust outlet to the sea if going offshore.