Certainly playing for England has helped Anderson but not to the extent that it is sometimes made out. - he's played 32% of the matches England played from his debut to the end of 2007 and then 84% of the matches since the start of 2008.
If he followed that trend (in terms of % of matches played) for West Indies as an example he'd have played
14 matches up until the end of 2007 and 93 after. Although even then 107 matches with a wicket per match ratio of 3.867 would give him 413 wickets. McGrath and Walsh would almost certainly be out of reach but he'd still only be 26 wickets behind Steyn as the 3rd highest wicket-taker ever as a seamer.
For Australia - he'd have played 137 matches with 529 wickets, 34 wickets away from having the most Test wickets ever as a seamer.
To take a closer look at New Zealand as they have been name-checked specifically I decided to look at his career up until the end of 2007 and 2008 onwards - looking at how often NZ played in home conditions and toured different countries and using how often Anderson plays (32% pre-2008, 84% onwards to calculate roughly how many games he would play in each country and then use Anderson's record in these countries to calculate how well he might do.
Until the end of 2007 using that same 32% average he'd have played 5 home matches, 2 matches in South Africa, 1 match in England*** and half a match in Bangladesh, Australia, India and Zimbabwe respectively. I have used his record in each country between 2003 and 2007 and worked from there - e.g. Anderson had 2/149 in 1 match in SA between 03 and 07 so as he would have had 2 matches in SA if playing for NZ I have just doubled that analysis to get 2 matches instead of just 1. In the case of Bangladesh where Anderson hasn't toured, I've used his first series which was 1 bowling innings in Sri Lanka as an attempted like-for-like replacement and added half a match for India (Anderson has favourable numbers there pre 2008 and you would presume Anderson would do well in Zimbabwe with the relatively weaker opposition). Not suggesting this is a perfect method but it tries to deal with fewer matches NZ play as well as the proportion of matches in different countries that NZ have.
First number = wickets most of these are long decimals as I am dividing to get wickets per match and then multiplying by number of matches he would have played..
2nd number = runs conceded - likewise I divided number of runs conceded by number of Tests and then multiplied by number of Tests Anderson may have been expected to play using the 32% matches played for pre-2008 and 84% for 2008 onwards. Final numbers required some rounding but tried to keep numbers relatively accurate at first stage so when collecting all the numbers for pre-2008 the overall average would be more accurate.
18.07/591.47 - home (England)
4/298 - South Africa
4/141.5 - New Zealand
0.5/63- SL - replacement for Bangladesh
6/79 - India and Zimbabwe
0.83/68.83 - Australia
33/1,242 = 33 wickets @ 37.64
*** As I am using Anderson's home stats when looking at NZ's home matches, I have used Anderson's stats in NZ for any matches NZ had in England.
2008 onwards where Anderson plays 84% of England's matches he would be expected to play 50 home matches, 8 Matches in Australia, 7 Matches in India, 6 Matches in England***, 5 Matches v Sri Lanka and Pakistan, 4 Matches in West Indies, 3 Matches in Bangladesh, South Africa and 2 matches in Zimbabwe. I have used his record in these countries from 2008 onwards. For Zimbabwe I have used Anderson's numbers v West Indies as a substitute as Anderson has good numbers there and you would expect him to get good returns v Zimbabwe and I have used Sri Lanka as a basis for Bangladesh again.
home (England) - 221.7/5010.5
Australia - 29.33/913.6
India - 17.5/640.5
NZ - 22.28/731.14
Sri Lanka - 16/347
Pakistan/UAE - 18.33/376.66
West Indies - 14.4/357.2
Bangladesh - 9.6/208.2
South Africa - 10.66/342.66
Zimbabwe - 7.2/178.6
367/9,106 = 367 wickets @ 24.81
This was just interesting to look at for me and we don't know if Anderson would do better or worse if he grew up in New Zealand its all speculation - maybe he doesn't have issues with people interfering with his actions/rotation becomes less necessary as New Zealand play fewer Tests anyway. Likewise, would he have adapted as well to a Kookaburra had he grown up in New Zealand we will just never know all we can say is he almost certainly would be better off if he was bowling with a Kookaburra ball all his life rather than somewhat learning on the job.
But for what its worth it looks like 400 wickets @ 25.87 is a fair estimation of Anderson's career if he played for New Zealand or at the least if he played with New Zealand's schedule. 9th most Test wickets by a fast bowler in history (ignoring Broad as that is only fair if you are going to raise the number of Tests England play) and 39 wickets away from Steyn in 3rd.
A look at things like this also ignores the fitness required - its fine England having all these games but that's also more reason for Anderson to have broken down at the end of the 2013/14 Ashes for example with a record of 336 wickets @ 30.70 and only played a couple more Tests with diminishing returns.
