The biggest error most beginners in Pattern make is flying too close to themselves. You need ROOM to do the maneuvers. The competition guidelines say your plane should be appx 250 to 300 ft away as it passes in front of the judges. The higher level competitors push the far edge of this range. All too often the beginners are less than 150 ft out... making themselves have to "scrunch" the maneuvers to keep inside the "box". If you are far enough out, there is TIME to establish a definite level line comming out of any centered maneuver before doing the turn-around maneuver. If you find you have to pop out of a centered manever immediately into the turn-around... you are flying WAY TOO close.

Most .40 size TRAINERS are capable of a reasonable performance at the Sportsman level of Pattern. There's not much inverted stuff. A well flown trainer can easilly beat a poorly flown "Pattern" aircraft at this level.

You need to practice with the plane you intend to compete with. You can ALSO practice with the other planes, and they can be used as "backup" planes in case of problems with your primary competition aircraft.

When practicing, have someone stand beside you calling the maneuvers. Have another person (or a few people) standing back 10 to 20 ft to observe and note what needs improvement. (the judges will be about 10 ft behind when you compete.) You, as the pilot, don't have the correct perspective to KNOW your loops are round without someone else's input. Pilots get a tunnel vision around the plane, and just wath that it smoothly changes its orientation. If the plane has a constant pitch angle change rate... the loop WON'T be round!

Make the maneuvers BIG. A loop should be at least 100 ft tall... 150 is better. Small, fast loops are very hard to make round. (and hard for the judges to grade)

Get the maneuver sequence down to where it is automatic. Don't confuse the "half reverse Cuban 8" with a "half Cuban 8." (do the right maneuver in the right order.)

At Sportsman level... do a lot of Cuban 8 practice. See how many Cuban 8's you can do, staying in the same place. (the half rolls all in the same spot.) You'll loose airspeed and fall out if you are making the 8's too small. A common .40 size trainer can do contiuous Cuban 8's until the pilot gets bored with them... or it runs out of gas.

For your trimming problem... visit http://www.nsrca.org/ They have a trimming guide. (gotta get the first trim in the list right BEFORE doing the next. They are all dependant on the one above being correct.) The better the plane is trimmed, the better it will perform.
Though I have seen an EXTREMELY good pilot take a plane which was completely out of kilter and make it look like it was on rails... there aren't many R/C pilots that are that good.