Maybe Ahkenazi Jews score somewhere between 0.3 and 1 SD higher on IQ tests or verbal IQ. The genetic explanation in the question (disease-conferring genes are also intelligence-boosting genes, there was no bottleneck, but directional selection) has come under criticism.
From your question it's unclear whether you want to know whether ethnic Jews are smarter for genetic reasons or simply smarter as a group. A commenter cites the article Natural History of Ashkenazi Intelligence, which makes a more specific claim.
An article critical of the genetic explanation by Prof. Brian Ferguson at Rutgers provides a useful counterperspective. It didn't appear in a peer-reviewed journal, but it did receive peer review and the author simply tells us why the editor rejected it, so it probably gives a good perspective on the debate.
He also cites one of the original authors with a revision to their theory, which may or may not be a bit more plausible.
Taking all the information together, it is fair to say that
most, though not all, studies give Ashkenazi descendants a higher IQ than non-Jewish whites.
How much? Take your pick.
Contrast his quote of the criticised paper:
Ashkenazi Jews have the highest average IQ of any ethnic group for which there are reliable data. They score 0.75 to 1.0 standard deviations above the general European average, corresponding to an IQ of 112-115 ... although a recent review concludes that the advantage is slightly less-only half a standard deviation (Cochran et al. 2006, 3)
His view (shortened):
The "recent review" they mention conveys a very different impression from that forceful assertion (Lynn 2004).
It has often been asserted that Jews have a higher average level of intelligence than non-Jewish whites of European origin ... Despite these assertions, the purported high IQ of the Jews has never been systematically reviewed... Despite the widespread consensus on the high Jewish verbal ability, not all studies have shown that Jews have a higher verbal IQ than gentiles. Furthermore, virtually all the existing studies are unsatisfactory because the
samples have been unrepresentative, very small or for other reasons (pgs. 201-202) ... There is only one study of the intelligence of American Jews in the last century which appears to be representative and had a reasonable sample size. This is Backman's (1972) analysis... The mean IQs of the Jew in relation to gentile white means of 100 and standard deviations of 15 were as follows: verbal knowledge (described as "a general factor, but primarily a measure of general information" ... )--107.8, English language 99.5; mathematics-1 09.7; visual reasoning ("a measure of reasoning with visual forms")-91.3; perceptual speed and accuracy - 102.2; memory (short term recall of verbal symbols)-95.1 (pg.203).
On exceptional ability (i.e. a fat tail at the upper end of the bell curve) he says:
NHAI also focuses on the high end of the curve, how many very, very gifted there are among Ashkenazim (Cochran et al. 2006, 661). In this they follow Patai (1977, 321-342) and others who tallied the disproportionate number of Jews winning Nobel Prizes. Cochran et al. add world chess champions, and winners of ACM Turing awards. This clear over-representation is cited frequently in print and internet discussions of NHAI, as clear demonstration of the power of
Jewish genes. But these impressive totals provide no evidence in favor of the NHAI hypothesis.
They would only support that conjecture if it were shown that a disproportionate number of those winners were heterozygote carriers of one of the supposed IQ-booster genes, as compared to the normal distribution of these alleles among the Ashkenazi.