Funny you asked. I just happen to have some data on that. The 1:7 was just a tad slower on average, until you look at the 55gr WWB data. I think the issue is that I needed a larger sample than 5 rounds per gun get any proof of concept. one outlier in 5 rounds could really skew the data. However, the trend is interesting:
Hornady TAP,75 gr, 16 inches, 1 in 9 twist
2590.0
2552.0
2557.0
2520.0
Avg velocity 2554.8
max variance 70.0
Hornady TAP, 75 gr, 16 inches, 1 in 7 twist
2513.0
2519.0
2529.0
Avg velocity 2520.3
max variance 16.0
Black Hills Reman., 75 gr, 16 inches, 1 in 9 twist
2549.0
2512.0
2445.0
2492.0
Avg velocity 2499.5
max variance 104.0
Black Hills Reman., 75 gr, 16 inches, 1 in 7 twist
2478.0
2460.0
2492.0
Avg velocity 2476.7
max variance 32.0
IMI M-193, 55 gr, 16 inches, 1 in 9 twist
3084.0
3062.0
3084.0
3067.0
Avg velocity 3074.3
max variance 22.0
IMI M-193, 55 gr, 16 inches, 1 in 7 twist
3017.0
3026.0
3079.0
3060.0
3052.0
Avg velocity 3046.8
max variance 62.0
Looks real convincing until you get here.
Winchester White Box, 55 gr, 16 inches, 1 in 9 twist
2975.0
2998.0
3000.0
2987.0
Avg velocity 2990.0
max variance 25.0
Winchester White Box, 55 gr, 16 inches, 1 in 7 twist
3000.0
2992.0
3042.0
3038.0
3023.0
Avg velocity 3019.0
max variance 50.0
I think you would need to test at least 20 rounds per gun to get a true picture.