You can try these settings and see what you think. These include the rounded corners and a little pincushion effect too. Keep in mind that I use these setting for vertical games so YMMV. I've found these settings don't seem to stress the system too much but still look decent. Check your FPS in mame and see if you are dropping frames or running less than 100%.
#
# DIRECT3D POST-PROCESSING OPTIONS
#
hlsl_enable 1
hlslpath hlsl
hlsl_prescale_x 0
hlsl_prescale_y 0
hlsl_preset -1
hlsl_write
hlsl_snap_width 1024
hlsl_snap_height 768
shadow_mask_alpha 0.1
shadow_mask_texture aperture.png
shadow_mask_x_count 800
shadow_mask_y_count 600
shadow_mask_usize 0.09375
shadow_mask_vsize 0.109375
curvature 0.040000
pincushion 0.02
scanline_alpha 1.0
scanline_size 1.0
scanline_height 1.0
scanline_bright_scale 1.0
scanline_bright_offset 0.0
scanline_jitter 0.05
defocus 0.0,0.0
converge_x 0.2,0.0,0.0
converge_y 0.0,0.0,0.0
radial_converge_x 0.2,0.0,0.0
radial_converge_y 0.0,0.0,0.0
red_ratio 1.0,0.0,0.0
grn_ratio 0.0,1.0,0.0
blu_ratio 0.0,0.0,1.0
saturation 1.15
offset -0.15,-0.16,-0.16
scale 1.15,1.16,1.16
power 1.1,0.95,0.8
floor 0.0,0.0,0.0
phosphor_life 0.0,0.0,0.0
#
# NTSC POST-PROCESSING OPTIONS
#
yiq_enable 0
yiq_cc 3.59754545
yiq_a 0.5
yiq_b 0.5
yiq_o 1.570796325
yiq_p 1.0
yiq_n 1.0
yiq_y 6.0
yiq_i 1.2
yiq_q 0.6
yiq_scan_time 52.6
yiq_phase_count 2
#
# VECTOR POST-PROCESSING OPTIONS
#
vector_length_scale 0.8
vector_length_ratio 500.0
#
# BLOOM POST-PROCESSING OPTIONS
#
vector_bloom_scale 0.3
raster_bloom_scale 0.225
bloom_lvl0_weight 1.0
bloom_lvl1_weight 0.21
bloom_lvl2_weight 0.19
bloom_lvl3_weight 0.17
bloom_lvl4_weight 0.15
bloom_lvl5_weight 0.14
bloom_lvl6_weight 0.13
bloom_lvl7_weight 0.12
bloom_lvl8_weight 0.11
bloom_lvl9_weight 0.10
bloom_lvl10_weight 0.09