Yeah it is dumb, been there, done that. First off I ended up quiting the job and having to take the money out of my 401K with penaty, but even if you don't you are going to be using after tax money to repay this, and when you retire you are going end paying tax and that amount again.
On your actual question. You don't provide enough information to tell. People say something like it is tax deductable and everyone jumps at 33% is what it will be lowered by, but that isn't true. If you deduct something it simply means that your income goes done by that much. If say you made $11,000, and could deduct $1,000 then your taxes are reduced by $1000 x what you pay in taxes, but if you are say in the "33% tax bracket" you are not paying 33%, because we have a system where the first X dollars aren't taxed, then the next tax bracket is tax at a lower percent then 33%, and on up to finally only some of you money is actually taxed at 33%. So you really need to know your effective tax rate which is approximately the tax you actually paid last year divided by your total income. That might be more like 15-20% and that is the number you need to reduce interest by to see what the real after tax rate is.
Also there is the effect of not having the money in the 401K. If your investments are doing better then the 9.5% minus the tax you are paying, then you are losing that money too, not just the difference between the two interest rates.
Of course if your investments in your 401K suck, maybe you paying yourself even after the taxes might be better.