Even good 15" tires will perform well, such as falken 452, t1r, f1s etc have very strong side walls and will not flex when being dragged sideways. Where did you get the doors cut and bar fitted?
As for suspension you have several choices depending on money, if you got the cash then get weitec etc, otherwise if you have a limited supply of money, some jom coilovers (as they are the stiffest out of the budget coilovers, such as fk), they were a vasy improvement on mk4.