The tearing is almost gone if you use pollux set file (put into the uae4all folder) and create a uae4all_pollux.gpe file with this text inside:
#!/bin/sh
./pollux_set 'lcd_timings=397,1,37,277,341,0,17,337;dpc_clkdiv0=9;ram_timings=2,9,4,1,1,1,1'
./uae4all.gpe
cd /usr/gp2x
exec...