On the cap, a recent BBC article said only one Premiership RU side made a profit last year. The difference in wealth of their owners compared to most of SL is a big factor - they can afford to pay to their much higher cap because their backers can afford it.
Until Douggie signed Fallon and Innes we were generally spectacularly bad at signing RU players in the 80s and 90s. Our sole Welsh signing was David Young, one of those RU props utterly unsuited to RL. Meanwhile some truly class Welshmen signed for other clubs - Devereux, Davies, Bateman, Gibbs, Ellis etc.