Sorry for delays, but a double double brew day is hindering me! I have had a look on beersmith and the IBU's are about 32 and the gravity is approx 1.050. so 4.8-5%.
If you use 2kg at the beginning instead of 1kg and use 32g all of boil and 15g instead of 10g for the last 15 mins the ibus remain the same.
heres the thing though the ratio between the maltyness and hoppyness for the original is 0.665. If you do the above, although the ibu (bitterness) stays the same the taste will be different as it is a stronger drink.
So you could also do this - 2kg instead of 1kg of malt at the beginning, 40g hops at start of boil and 25g in the last 15 mins will keep the same ratio, so in theroy a similar taste....maybe!
The choice is yours, Ive never upped the percentage of a beer I liked, but the 2 methods above is how I would do it.
Id also wait for some more opinions from extract brewers as I am all grain so I dont tend to use beersmith for extract recipes and someone else may be able to sort it out better for you.
Hope it sort of helps, I now it may be a bit confusing so keep asking and I will post to see if some extract brewers can help us out