Ok, first I respectfully disagree about being able to pull the sails in. The 645MG will have no problems. However, there are some things that you can do to offset the tendancy to 'round up' (your bow starts heading directly into the wind without you being able to control it).

1. Flatten the pocket in the bottom of the main sail. You can sometimes do this by tightening a line on the main boom at the back end the will pull the back end of the sail back. Exactly what this looks like on your boat I'm not sure, as it varies from boat to boat. More pocket (check this by laying the boat on it's side with the sails hauled all the way in AT HOME out of the wind. You will notice there will likely be a gap between the boom and the sail foot, or bottom. This is called your mainsail camber. The more camber, the more power your sails have, and in strong winds you need to de-power, so doing this will help to an extent.

2. Adjust your boomvang. What this is, is the line going diagonal from approximately the bottom of the mast, to about 3-4 inches in back of the mast on the main boom. This controls (hopefully on your boat it's adjustable) how much of an angle your boom is at. In most moderate or lighter conditions, the boom should be level with the deck (or parallel). However, in stronger winds, you can loosen this allowing the back end of the boom to point more 'up'. Why do this? Well, it puts what's called 'twist' in the sail, which allows the air to spill (And again, de-power) off the sail a little. I hope this description makes sense, but it's hard to put into words--easier to show.

3. You can make your mast tilt more forward (the top of it) which is called 'raking' it. Without getting too technical, it also helps to combat weather helm. On most boats this is done by loosening the backstay (the line/wire leading from the top of the mast to the back of the boat) and at the same time tightening the forestay--the line that leads from the top/front of the mast down to the jib sail.

4. Once you get your mast rake and boom adjusted, then tighten the backstay up. This flattens the top of the sail, which once again spills air, depowering the boat.

Any of these, or a combination of these can help the boat handle better. The uninitiated would probably be suprised that there are so many tuning items on a 'simple' RC sailboat. I can tell you that with experimentation of these (don't overdo them, and remember that one change often affects other aspects, so other adjustments must be made to balance things) over time you can learn to sail your boat in lots of different conditions making the most of what the wind gives you. I learned this through racing in my club and elsewhere, but it's handy to know for simple day sailing too.

I really am not trying to confuse you. I hope some of this helps at least. However, the stronger servo will help you haul in the sails, but it will matter very little if your boat isn't set up right for the conditions.

Good luck and happy sailing!

Andy