I can't comment on current availability in gunshops, but Edgar Brothers has been importing IMR-4451 for over two years now. I tried it in the 6.5 Creedmoor in a side by side test against H4350 in a Savage 12 LRP in March 2016 using Norma brass, CCI-BR2s and the 140gn Nosler Custom Comp HPBT (which the rifle liked a lot).
In this cartridge, 4451 seems to be a little slower burning than H4350 - at 40gn for instance, it produced 45 fps lower MV with the 140gn Hornady AMax. Having looked at Hodgdon's online data for various cartridges, maximum charges at near SAAMI maximum pressures are close to those for H4350 but vary according the cartridge and calibre - in some applications it appears to be marginally faster burning; in others a little slower.
It is an excellent propellant, very clean burning, and has an anti-copper additive in its formulation. As with all the new IMR 'Endurons', it is mildly double-based, something under 10% of the recipe by weight being nitroglycerin. This is apparently a trend these days as ADI/Hodgdon aside most recent introductions are of this form.
I'm awaiting the next slowest burning Enduron grade being cleared for Reach and imported, the more recently introduced IMR-4955 that is the Reach compliant equivalent to IMR and Hodgdon 4831. We now have a gap for such a grade with the loss of the IMR/H versions. On paper (burning rate charts) Viht N160 is there - but it's not in practice being a considerably quicker powder and prone to pressure spiking as full pressures are achieved. The rather high nitroglycerin content N560 is in the right ballpark being considerably slower despite Vihtavuori showing 160/560 as having the same burning speeds, and Reload Swiss RS70 is a contender, but I suspect it may be hard on barrels if loaded up to full pressures as per its quicker RS60 stablemate. Nitrochemie is allegedly investigating a single-based product to fill this gap - ie a slower burning version of RS62 and samples for review were promised for early this year - but no sign of them to date.
FWIW (not a lot, every rifle / barrel being a little different), the best results I got with the Savage were with RS62 at a modest MV (2,710 fps) with the Nosler 140 in Norma brass - it shot well enough to get me a second place in 'factory class' in a UK BR Association 1,000 benchrest match with the smallest of the four 5-round groups being 7.087". This was despite the LRP being very poorly suited to single loading rounds (mandatory in BR competition) each one having to have the bullet tip wiggled into the chamber before closing the bolt and hence giving very slow shooting. (The norm in long-range benchrest is to choose a wind and/or mirage condition that might last then get the shots off very fast - 20 seconds or less for competitors with specialist actions - hoping that there isn't a wind change during the string.)