[boxs2000]

Your vibration and shimmy could be caused by a few things. This is what I would check first before you blame it on bad balancing. Presuming it was properly balanced;

1st check to make sure you are using the proper lug nuts. Jack the car up, remove the lug nuts and look at the tapper on the the ends, if I remember correctly, the o.e. Porsche is a rounded taper, make sure it matches your new rims. Some rims have a strait taper some are rounded depending on the brand and style.

2nd while you have it Jacked up, remove the wheel check to see if the new rims are "hub Centric" What that means is, just like your stock or o.e rims, when you remove the lug nuts the rim fits snugly around the hub of the car.

3rd make sure the direction of the tires are pointing in the way .(directional tires)

I don't think it is an alignment issue, a bad alignment usually causes poor handling and bad/un-even tire wear. I do recommend you get a new alignment since you have new tires and the way the car is going to stand will be different. Its always a good idea to get re aligned when you get new tires.

You did not mention what brand your new rims are. Not to imply that your new wheels are of lower quality, but many after market wheels that are not Porsche or brand specific tend to come in many fits all vehicle configuration which is why they make them that way to be able to mount on many different cars instead of one specific brand.