I agree. You can use the same algorithm for computing both. But Arabic has better constants. Because it's a discrete convolution instead of grouping and aggregating.
1 6
1 1 6
7 7 42
1, 6+7, 42 = 100 + 130 + 42 = 272
Same process in Roman
X V I
X C L X
X C L X
I X V I
I X V I
I X V I
CC LL(=C) XXXXX(=L) VVV(=XV) III = CCCLXVIII = 16*23=368
This supports the arguments that Arabic numbers really are better suited for things like multiplying. They don't have the property that multiplication is convolution, so you can't even do things like truncate your computation to get an approximation.
There's probably a way to formalize this with an entropy argument: that roman numerals are inefficient encoding. Because given some n-length string of numerals, firstly many are invalid encodings, and secondly among the valid numerals there isn't a uniform distribution from strings to integers. Something like that.